Optical switch, optical amplifier and optical power controller as well as optical add-drop multiplexer

ABSTRACT

The first present invention provides an optical switch including the following elements. At least a plurality of optical transmission lines are provided for transmissions of optical signals. Each of the at least plurality of optical transmission lines have at least an impurity doped fiber. At least an excitation light source is provided for emitting an excitation light. At least an excitation light switch is provided which is connected to the excitation light source and also connected to the at least plurality of optical transmission lines for individual switching operations to supply the excitation light to the at least plurality of optical transmission lines to feed the excitation light to the impurity doped fiber on the at least plurality of optical transmission lines, thereby causing an excitation of the impurity doped fiber on selected one of the at least plurality of optical transmission lines so as to permit a transmission of the optical signal through the excited impurity doped fiber, whilst unselected one of the impurity doped fibers is unexcited whereby the optical signals are absorbed into the unselected one of the impurity doped fibers thereby to discontinue transmission of the optical signal by the unselected one of the impurity doped fibers.

This application is a division of application Ser. No. 09/939,665, filedon Aug. 28, 2001, now U.S. Pat. No. 6,466,344 which is a division ofapplication Ser. No. 09/181,620, filed on Oct. 28, 1998, now U.S. Pat.No. 6,424,440 the entire contents of which are hereby incorporated byreference

BACKGROUND OF THE INVENTION

The present invention relates to an optical switch, an optical amplifierand an optical power controller as well as an optical add-dropmultiplexer.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a noveloptical switch free from the above problems.

It is a further object of the present invention to provide a noveloptical amplifier.

It is a still further object of the present invention to provide a noveloptical power controller.

It is yet a further object of the present invention to provide a noveloptical add-drop multiplexer.

The first present invention provides an optical switch including thefollowing elements. At least a plurality of optical transmission linesare provided for transmissions of optical signals. Each of the at leastplurality of optical transmission lines have at least an impurity dopedfiber. At least an excitation light source is provided for emitting anexcitation light. At least an excitation light switch is provided whichis connected to the excitation light source and also connected to the atleast plurality of optical transmission lines for individual switchingoperations to supply the excitation light to the at least plurality ofoptical transmission lines to feed the excitation light to the impuritydoped fiber on the at least plurality of optical transmission lines,thereby causing an excitation of the impurity doped fiber on selectedone of the at least plurality of optical transmission lines so as topermit a transmission of the optical signal through the excited impuritydoped fiber, whilst unselected one of the impurity doped fibers isunexcited whereby the optical signals are absorbed into the unselectedone of the impurity doped fibers thereby to discontinue transmission ofthe optical signal by the unselected one of the impurity doped fibers.

The above and other objects, features and advantages of the presentinvention will be apparent from the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments according to the present invention will bedescribed in detail with reference to the accompanying drawings.

FIG. 1 is a diagram illustrative of a first novel optical switch havinga single input and two outputs in a first embodiment in accordance withthe present invention.

FIG. 2 is a diagram illustrative of a second novel optical switch havinga single input and two outputs in a second embodiment in accordance withthe present invention.

FIG. 3 is a diagram illustrative of a third novel optical switch havinga single input and two outputs in a third embodiment in accordance withthe present invention.

FIG. 4 is a diagram illustrative of a fourth novel optical switch havinga single input and two outputs in a fourth embodiment in accordance withthe present invention.

FIG. 5 is a diagram illustrative of a fifth novel optical switch havingtwo inputs and two outputs in a fifth embodiment in accordance with thepresent invention.

FIG. 6 is a diagram illustrative of a sixth novel optical switch havingtwo inputs and two outputs in a sixth embodiment in accordance with thepresent invention.

FIG. 7 is a diagram illustrative of a seventh novel optical switchhaving four separate optical transmission lines for separately switchingoptical signal transmissions on the four separate optical transmissionlines in a seventh embodiment in accordance with the present invention.

FIG. 8 is a diagram illustrative of an eighth novel optical switchhaving four separate optical transmission lines for separately switchingoptical signal transmissions on the four separate optical transmissionlines in an eight embodiment in accordance with the present invention.

FIG. 9 is a diagram illustrative of a ninth novel optical switchprovided in a novel first optical add-drop multiplexer performingoptical addition, drop and transmission of said optical signals in aninth embodiment in accordance with the present invention.

FIG. 10 is a diagram illustrative of a tenth novel optical switch as anoptical gate switch in a tenth embodiment in accordance with the presentinvention.

FIG. 11 is a schematic view illustrative of an integration of the abovenovel optical gate switch in a tenth embodiment in accordance with thepresent invention.

FIG. 12 is a diagram illustrative of a novel optical add-dropmultiplexer using an optical gate switch of FIG. 10 for performingoptical addition, drop and transmission of said optical signals in aneleventh embodiment in accordance with the present invention.

FIG. 13 is a schematic view illustrative of an integration of the abovenovel optical add-drop multiplexer in an eleventh embodiment inaccordance with the present invention.

FIG. 14 is a diagram illustrative of a novel wavelength-multiplexedoptical add-drop multiplexer using four sets of the above novel opticaladd-drop multiplexer of FIG. 9 in a twelfth embodiment in accordancewith the present invention.

FIG. 15 is a diagram illustrative of a novel wavelength-multiplexedoptical add-drop multiplexer using four sets of the above novel opticaladd-drop multiplexer of FIG. 12 in a thirteenth embodiment in accordancewith the present invention.

FIG. 16 is a diagram illustrative of a novel wavelength-multiplexedoptical add-drop multiplexer having four looped optical transmissionpaths in a fourteenth embodiment in accordance with the presentinvention.

FIG. 17 is a diagram illustrative of the first opticalmultiplexer/demultiplexer used in the wavelength-multiplexed opticaladd-drop multiplexer of FIG. 16 in a fourteenth embodiment in accordancewith the present invention.

FIG. 18 is a diagram illustrative of a novel wavelength-multiplexedoptical add-drop multiplexer having four looped optical transmissionpaths in a fifteenth embodiment in accordance with the presentinvention.

FIG. 19 is a diagram illustrative of a novel wavelength-multiplexedoptical amplifier having four looped optical transmission paths in asixteenth embodiment in accordance with the present invention.

FIG. 20 is a diagram illustrative of a novel wavelength-multiplexedoptical add-drop multiplexer having four looped optical transmissionpaths in a seventeenth embodiment in accordance with the presentinvention.

FIG. 21 is a diagram illustrative of a novel optical gate switchutilizing optical wavelength multiplexer/demultiplexer and an erbiumdoped fiber in an eighteenth embodiment in accordance with the presentinvention.

FIG. 22 is a diagram illustrative of a novel optical gate switchutilizing optical wavelength multiplexer/demultiplexer and an erbiumdoped fiber in an nineteenth embodiment in accordance with the presentinvention.

FIG. 23 is a diagram illustrative of a novel optical gate switchutilizing optical wavelength multiplexer/demultiplexer and an erbiumdoped fiber in an twenty embodiment in accordance with the presentinvention.

FIG. 24 is a novel wavelength-multiplexed optical add-dropmultiplexer/demultiplexers in place of optical couplers and furtherutilizing erbium doped fibers in a twenty first embodiment in accordancewith the present invention.

FIG. 25 is a novel wavelength-multiplexed optical add-drop multiplexerwhich is modified from the above novel wavelength-multiplexed opticaladd-drop multiplexer of FIG. 15 by utilizing opticalmultiplexer/demultiplexers in place of optical couplers and furtherutilizing erbium doped fibers in a twenty second embodiment inaccordance with the present invention.

FIG. 26 is a novel wavelength-multiplexed optical add-drop multiplexerwhich is modified from the above novel wavelength-multiplexed opticaladd-drop multiplexer of FIG. 16 by utilizing opticalmultiplexer/demultiplexers in place of optical couplers and furtherutilizing erbium doped fibers in a twenty third embodiment in accordancewith the present invention.

FIG. 27 is a novel wavelength-multiplexed optical add-drop multiplexerutilizing optical multiplexer/demultiplexers in place of opticalcouplers and further utilizing erbium doped fibers in a twenty fourthembodiment in accordance with the present invention.

FIG. 28 is a novel wavelength-multiplexed optical add-drop multiplexerwhich is modified from the above novel wavelength-multiplexed opticaladd-drop multiplexer of FIG. 15 by utilizing opticalmultiplexer/demultiplexers in place of optical couplers and furtherutilizing erbium doped fibers in a twenty fifth embodiment in accordancewith the present invention.

FIG. 29 is a novel wavelength-multiplexed optical add-drop multiplexerwhich is modified from the above novel wavelength-multiplexed opticaladd-drop multiplexer of FIG. 16 by utilizing opticalmultiplexer/demultiplexers in place of optical couplers and furtherutilizing erbium doped fibers in a twenty sixth embodiment in accordancewith the present invention.

DISCLOSURE OF THE INVENTION

The first present invention provides an optical switch including thefollowing elements. At least a plurality of optical transmission linesare provided for transmissions of optical signals. Each of the at leastplurality of optical transmission lines have at least an impurity dopedfiber. At least an excitation light source is provided for emitting anexcitation light. At least an excitation light switch is provided whichis connected to the excitation light source and also connected to the atleast plurality of optical transmission lines for individual switchingoperations to supply the excitation light to the at least plurality ofoptical transmission lines to feed the excitation light to the impuritydoped fiber on the at least plurality of optical transmission lines,thereby causing an excitation of the impurity doped fiber on selectedone of the at least plurality of optical transmission lines so as topermit a transmission of the optical signal through the excited impuritydoped fiber, whilst unselected one of the impurity doped fibers isunexcited whereby the optical signals are absorbed into the unselectedone of the impurity doped fibers thereby to discontinue transmission ofthe optical signal by the unselected one of the impurity doped fibers.

It is preferable that the optical switch further includes: a singleinput side optical transmission line; and a single input side opticalcoupler connected to the single input side optical transmission line,and wherein the at least plurality of optical transmission linescomprise first and second optical transmission lines which are connectedthrough the single input side optical coupler to the single input sideoptical transmission line, and the first and second optical transmissionlines have first and second impurity doped fibers, and wherein the atleast excitation light source comprises a single excitation lightsource, and the at least excitation light switch comprises a singleexcitation light switch which has first and second output terminals forselecting any one of the first and second output terminals, and thefirst output terminal is connected through a first optical coupler tothe first impurity doped first to feed the excitation light to the firstimpurity doped fiber only when the first output terminal is selected bythe single excitation light switch, and the second output terminals isconnected through a second optical coupler to the second impurity dopedfiber to feed the excitation light to the second impurity doped fiberonly when the second output terminal is selected by the singleexcitation light switch.

It is preferable further comprise first and second optical filers. Thefirst optical filter is provided on the first optical transmission lineand positioned between the first optical coupler and an output terminalof the first optical transmission line so as to remove a noise from thefirst optical signal when the first impurity doped fiber is excited. Thesecond optical filter is provided on the second optical transmissionline and positioned between the second optical coupler and an outputterminal of the second optical transmission line so as to remove a noisefrom the second optical signal when the second impurity doped fiber isexcited.

It is preferable further comprise the following elements. A firstoptical reflective mirror is provided on one end of the first opticaltransmission line for reflecting the first optical signal passed throughthe first impurity doped fiber excited so that the reflected firstoptical signal is again transmitted through the first impurity dopedfiber excited to an opposite end as an output terminal of the firstoptical transmission line. A first optical isolator is provided betweenthe input side optical coupler and the first optical transmission linefor permitting a unidirectional transmission of an optical signal fromthe input side optical coupler to the first optical transmission line. Asecond optical reflective mirror is provided on one end of the secondoptical transmission line for reflecting the second optical signalpassed through the second impurity doped fiber excited so that thereflected second optical signal is again transmitted through the secondimpurity doped fiber excited to an opposite end as an output terminal ofthe second optical transmission line. A second optical isolator isprovided between the input side optical coupler and the second opticaltransmission line for permitting a unidirectional transmission of anoptical signal from the input side optical coupler to the second opticaltransmission line.

It is preferable further comprise the following elements. A firstoptical reflective mirror is provided on one end of the first opticaltransmission line for reflecting the first optical signal passed throughthe first impurity doped fiber excited so that the reflected firstoptical signal is again transmitted through the first impurity dopedfiber excited to an opposite end as an output terminal of the firstoptical transmission line. A second optical reflective mirror isprovided on one end of the second optical transmission line forreflecting the second optical signal passed through the second impuritydoped fiber excited so that the reflected second optical signal is againtransmitted through the second impurity doped fiber excited to anopposite end as an output terminal of the second optical transmissionline. A circulator is provided as the input side optical coupler and anoptical isolator provided between the input side optical transmissionline and the first and second optical transmission lines.

It is preferable that the first optical coupler is inserted between thefirst impurity doped fiber and an output terminal of the first opticaltransmission line so as to feed the excitation light to the firstimpurity doped fiber in an opposite direction to a transmission of thefirst optical signal through the first impurity doped fiber excited, andalso the second optical coupler is inserted between the second impuritydoped fiber and an output terminal of the second optical transmissionline so as to feed the excitation light to the second impurity dopedfiber in an opposite direction to a transmission of the second opticalsignal through the second impurity doped fiber excited.

It is preferable that the first optical coupler is inserted between thefirst impurity doped fiber and the input side optical coupler so as tofeed the excitation light to the first impurity doped fiber in the samedirection as a transmission of the first optical signal through thefirst impurity doped fiber excited, and also the second optical coupleris inserted between the second impurity doped fiber and the input sideoptical coupler so as to feed the excitation light to the secondimpurity doped fiber in the same direction as a transmission of thesecond optical signal through the second impurity doped fiber excited.

It is preferable that the optical switch has two inputs and two outputsand comprises a pair of first and second optical switches connected toeach other through at least an interconnecting optical transmissionline, and wherein each of the first and second optical switches furthercomprises the following elements. A single input side optical coupler isprovided which is connected to the single input side opticaltransmission line. First and second optical transmission lines areconnected through the single input side optical coupler to the singleinput side optical transmission line. The first and second opticaltransmission lines have first and second impurity doped fibers. A singleexcitation light source is provided. A single excitation light switch isprovided which has first and second output terminals for selecting anyone of the first and second output terminals. The first output terminalis connected through a first optical coupler to the first impurity dopedfiber to feed the excitation light to the first impurity doped fiberonly when the first output terminal is selected by the single excitationlight switch. The second output terminal is connected through a secondoptical coupler to the second impurity doped fiber to feed theexcitation light to the second impurity doped fiber only when the secondoutput terminal is selected by the single excitation light switch.

It is preferable that each of the first and second optical switchesfurther comprises first and second optical filters. The first opticalfilter is provided on the first optical transmission line and positionedbetween the first optical coupler and an output terminal of the firstoptical transmission line so as to remove a noise from the first opticalsignal when the first impurity doped fiber is excited. The secondoptical filter is provided on the second optical transmission line andpositioned between the second optical coupler and an output terminal ofthe second optical transmission line so as to remove a noise from thesecond optical signal when the second impurity doped fiber is excited.

It is preferable that each of the first and second optical switchesfurther comprises the following elements. A first optical reflectivemirror is provided on one end of the first optical transmission line forreflecting the first optical signal passed through the first impuritydoped fiber excited so that the reflected first optical signal is againtransmitted through the first impurity doped fiber excited to anopposite end as an output terminal of the first optical transmissionline. A first optical isolator is provided between the input sideoptical coupler and the first optical transmission line for permitting aunidirectional transmission of an optical signal from the input sideoptical coupler to the first optical transmission line. A second opticalreflective mirror is provided on one end of the second opticaltransmission line for reflecting the second optical signal passedthrough the second impurity doped fiber excited so that the reflectedsecond optical signal is again transmitted through the second impuritydoped fiber excited to an opposite end as an output terminal of thesecond optical transmission line. A second optical isolator is providedbetween the input side optical coupler and the second opticaltransmission line for permitting a unidirectional transmission of anoptical signal from the input side optical coupler to the second opticaltransmission line.

It is preferable that each of the first and second optical switchesfurther comprises the following elements. A first optical reflectivemirror is provided on one end of the first optical transmission line forreflecting the first optical signal passed through the first impuritydoped fiber excited so that the reflected first optical signal is againtransmitted through the first impurity doped fiber excited to anopposite end as an output terminal of the first optical transmissionline. A second optical reflective mirror is provided on one end of thesecond optical transmission line for reflecting the second opticalsignal passed through the second impurity doped fiber excited so thatthe reflected second optical signal is again transmitted through thesecond impurity doped fiber excited to an opposite end as an outputterminal of the second optical transmission line. A circulator isprovided as the input side optical coupler and an optical isolatorprovided between the input side optical transmission line and the firstand second optical transmission lines.

It is preferable that, for each of the first and second opticalswitches, the first optical coupler is inserted between the firstimpurity doped fiber and an output terminal of the first opticaltransmission line so as to feed the excitation light to the firstimpurity doped fiber in an opposite direction to a transmission of thefirst optical signal through the first impurity doped fiber excited, andalso the second optical coupler is inserted between the second impuritydoped fiber and an output terminal of the second optical transmissionline so as to feed the excitation light to the second impurity dopedfiber in an opposite direction to a transmission of the second opticalsignal through the second impurity doped fiber excited.

It is preferable that, for each of the first and second opticalswitches, the first optical coupler is inserted between the firstimpurity doped fiber and the input side optical coupler so as to feedthe excitation light to the first impurity doped fiber in the samedirection as a transmission of the first optical signal through thefirst impurity doped fiber excited, and also the second optical coupleris inserted between the second impurity doped fiber and the input sideoptical coupler so as to feed the excitation light to the secondimpurity doped fiber in the same direction as a transmission of thesecond optical signal through the second impurity doped fiber excited.

It is preferable that the optical switch has two inputs and two outputsand comprises a pair of first and second optical switches connected toeach other through at least an interconnecting optical transmissionline, and a common excitation light source connected to the first andsecond optical switches, and wherein each of the first and secondoptical switches further comprises the following elements. A singleinput side optical coupler is provided which is connected to the singleinput side optical transmission line. First and second opticaltransmission lines are provided which are connected through the singleinput side optical coupler to the single input side optical transmissionline. The first and second optical transmission lines have first andsecond impurity doped fibers. A single excitation light switch isprovided which is connected to the common excitation light source, thesingle excitation light switch having first and second output terminalsfor selecting any one of the first and second output terminals, and thefirst output terminal being connected through a first optical coupler tothe first impurity doped fiber to feed the excitation light to the firstimpurity doped fiber only when the first output terminal is selected bythe single excitation light switch, and the second output terminal beingconnected through a second optical coupler to the second impurity dopedfiber to feed the excitation light to the second impurity doped fiberonly when the second output terminal is selected by the singleexcitation light switch.

It is preferable that the at least plurality of optical transmissionlines are separated from each other for separate transmission ofdifferent optical signals on the plurality of separated opticaltransmission lines. Each of the separated optical transmission lines hasa single impurity doped fiber. The at least excitation light sourcecomprises a single excitation light source. The at least excitationlight switch comprises a single excitation light switch for separateswitching operations to the at least plurality of optical transmissionlines to separately control individual excitations of the impurity dopedfibers on the least plurality of optical transmission lines.

It is preferable that the at least plurality of optical transmissionlines are separated from each other for separate transmission ofdifferent optical signals on the plurality of separated opticaltransmission lines, and each of the separated optical transmission lineshas a single impurity doped fiber, and the at least excitation lightsource comprises two excitation light source, and further at leastexcitation light switch comprises a single optical cross connector forseparate switching operations to the at least plurality of opticaltransmission lines to separately control individual excitations of theimpurity doped fibers on the at least plurality of optical transmissionlines.

The second present invention provides an optical switch comprising thefollowing elements. A first optical transmission line is provided fortransmitting a first optical signal. An optical reflectivity variablemirror is provided which is capable of varying a reflectivity in a rangeof 0% to 100% for reflecting the first optical signal. The opticalreflectivity variable mirror is connected with the first opticaltransmission line. A second optical transmission line is provided whichis connected through the optical reflectivity variable mirror to thefirst optical transmission line. An optical transmitter is providedwhich is connected through the second optical transmission line to theoptical reflectivity variable mirror for transmitting a second opticalsignal. If the optical reflectivity variable mirror sets thereflectivity at less than 100%, then the first optical signal isreflected by the optical reflectivity variable mirror so that the firstoptical signal is outputted from the first optical transmission line, ifthe optical reflectivity variable mirror sets the reflectivity at 100%,then the first optical signal is transmitted through the opticalreflectivity variable mirror, whilst the second optical signaltransmitted from the optical transmitter is also transmitted through theoptical reflectivity variable mirror to be outputted from the firstoptical transmission line.

The third present invention provides an optical add-drop multiplexercomprising at least a single set of the following elements. A firstoptical transmission line is provided for transmitting a first opticalsignal. An optical coupler is provided on the first optical transmissionline for dividing the first optical signal into first and second dividedoptical signals. A fourth optical transmission line is provided which isconnected with the optical coupler for transmitting the first dividedoptical signal. An optical receiver is provided which is connectedthrough the fourth optical transmission line to the optical coupler forreceiving the first divided optical signal. An optical reflectivityvariable mirror is provided which is capable of varying a reflectivityin a range of 0% to 100%. The optical reflectivity variable mirror isconnected with first optical transmission line for reflecting the seconddivided optical signal. A second optical transmission line is providedwhich is connected through the optical reflectivity variable mirror tothe first optical transmission line. An optical transmitter is providedwhich is connected through the second optical transmission line to theoptical reflectivity variable mirror for transmitting a second opticalsignal. If the optical reflectivity variable mirror sets thereflectivity at less than 100%, then the first optical signal isreflected by the optical reflectivity variable mirror so that the firstoptical signal is outputted from the first optical transmission line. Ifthe optical reflectivity variable mirror sets the reflectivity at 100%,then the first optical signal is transmitted through the opticalreflectivity variable mirror, whilst the second optical signaltransmitted from the optical transmitter is also transmitted through theoptical reflectivity variable mirror to be outputted from the firstoptical transmission line.

It is preferable that the optical add-drop multiplexer comprises aplurality of the optical add-drop multiplexers, and further comprisingan optical device having at least any one of multiplexing function anddemultiplexing function so that the optical add-drop multiplexers areoperable to different wavelength optical signals.

The fourth present invention provides an optical add-drop multiplexercomprising at least a single set of the following elements. An inputside optical transmission line is provided for transmitting a firstoptical signal. An input side optical coupler is provided on the firstoptical transmission line for dividing the first optical signal intofirst and second divided optical signals. First and second opticaltransmission lines are provided which are connected with the input sideoptical coupler for transmissions of the first and second dividedoptical signals respectively. The first and second optical transmissionlines have first and second impurity doped fibers. An optical receiveris provided which is connected through the first optical transmissionline to the first impurity doped fiber for receiving the first dividedoptical signal only when the first impurity doped fiber is excited. Anoptical transmitter is provided which is connected through the secondoptical transmission line to the second impurity doped fiber fortransmitting a second optical signal through the second impurity dopedfiber to the input side optical transmission line for output of thesecond optical signal only when the second impurity doped fiber isexcited. At least an excitation light source is provided for emitting anexcitation light. An excitation light switch is provided which isconnected to the excitation light source and also connected to the firstand second optical transmission lines for selective switching operationsto supply the excitation light to any one of the first and secondoptical transmission lines to feed the excitation light to selected oneof the first and second impurity doped fibers, thereby causing anexcitation of the selected one of the first and second impurity dopedfibers, whilst unselected one of the first and second impurity dopedfibers is unexcited.

It is preferable that the optical add-drop multiplexer comprises aplurality of the optical add-drop multiplexers, and further comprisingan optical device having at least any one of multiplexing function anddemultiplexing function so that the optical add-drop multiplexers areoperable to different wavelength optical signals.

The fifth present invention provides an optical gate switch comprisingthe following elements. A first optical transmission line is providedfor transmitting an input optical input signal. A second opticaltransmission line is provided for transmitting an optical output signal.A fourth optional transmission line is connected through an opticalcoupler to both the first and second optional transmission lines. Thefourth optional transmission line has at least a impurity doped fiberand a wavelength band selective optical reflecting mirror capable ofselecting a wavelength band of a light to be reflected. The impuritydoped fiber is positioned between the wavelength band selective opticalreflecting mirror. An excitation light source is provided which isconnected to the wavelength band selective optical reflecting mirror forcontrolling an emission of an excitation light so that if the excitationlight source emits the excitation light to feed the excitation light tothe impurity doped fiber so as to excite the impurity doped fiber,whereby the optical input signal is transmitted through the excitedimpurity doped fiber and amplified by the excited impurity doped fiberand subsequently the amplified optical signal is reflected by thewavelength band selective optical reflecting mirror before the reflectedoptical signal is then transmitted through the excited impurity dopedfiber and further amplified by the excited impurity doped fiber forsubsequent output of the further amplified optical signal through theoutput signal optical transmission line.

The sixth present invention provides an optical add-drop multiplexercomprising at least a single set of the following elements. A firstoptional transmission line is provided for transmitting an input opticalinput signal. A second optical transmission line is provided fortransmitting an optical output signal. A fourth optional transmissionline is provided which is connected through an optical coupler to boththe first and second optional transmission lines. The fourth optionaltransmission line has at least a impurity doped fiber and a wavelengthband selective optical reflecting mirror capable of selecting awavelength band of a light to be reflected. The impurity doped fiber ispositioned between the wavelength band selective optical reflectingmirror. An optical receiver is provided which is connected through asecond optical coupler to the fourth optical transmission line so thatthe second optical coupler is positioned between the first opticalcoupler and the impurity doped fiber for allowing the optical receiverreceives a part of the optical input signal. An optical transmitter isprovided which is connected through a fourth optical coupler to theoutput signal transmission line for transmitting a second optical signalas a substitute output signal only when no output signal is suppliedfrom the impurity doped fiber. An excitation light source is providedwhich is connected to the wavelength band selective optical reflectingmirror for controlling an emission of an excitation light so that if theexcitation light source emits the excitation light to feed theexcitation light to the impurity doped fiber so as to excite theimpurity doped fiber, whereby the optical input signal is transmittedthrough the excited impurity doped fiber and amplified by the excitedimpurity doped fiber and subsequently the amplified optical signal isreflected by the wavelength band selective optical reflecting mirrorbefore the reflected optical signal is then transmitted through theexcited impurity doped fiber and further amplified by the excitedimpurity doped fiber for subsequent output of the further amplifiedoptical signal through the output signal optical transmission line.

It is preferable that the optical add-drop multiplexer comprises aplurality of the optical add-drop multiplexers, and further comprisingan optical device having at least any one of multiplexing function anddemultiplexing function so that the optical add-drop multiplexers areoperable to different wavelength optical signals.

The seventh present invention provides an optical transmission linejunction structure comprising at least three optical transmission linesfor transmuting optical signals and an optical device having at leastany one of wavelength multiplexing and demultiplexing functionsconnected to the at least three optical transmission lines, so that theoptical device having at least any one of multiplexing anddemultiplexing functions serves as a same roll as an optical coupler soas to reduce an optical power loss when the optical signal istransmitted through the optical transmission line junction structure.

It is preferable that the optical device comprises an opticalmultiplexer/demultiplexer.

It is preferable that the optical device comprises an opticalmultiplexer.

It is preferable that the optical device comprises an opticaldemultiplexer.

The eighth present invention provides an optical transmission linejunction structure comprising at least three optical transmission linesfor transmuting optical signals and an optical circulator connected tothe at least three optical transmission lines, so that the opticalcirculator serves as a same roll as an optical coupler so as to reducean optical power loss when the optical signal is transmitted through theoptical transmission line junction structure.

The ninth present invention provides an optical loop-structured circuithaving at least a plurality of looped optical transmission lines havingat least a plurality of optical transmission line junctions from whichat least three optical transmission lines extend, wherein at least oneof the plurality of optical transmission line junctions has an opticaldevice having at least any one of wavelength multiplexing anddemultiplexing functions, which is connected to the at least threeoptical transmission lines, so that the optical device having at leastany one of multiplexing and demultiplexing functions serves as a sameroll as an optical coupler so as to reduce an optical power loss whenthe optical signal is transmitted through the optical transmission linejunction structure.

It is preferable that all of the plurality of optical transmission linejunctions have the optical devices.

It is preferable that at least one of the plurality of looped opticaltransmission lines has at least a single set of an optical amplifier andan optical isolator so that the optical loop-structured circuit has afunction of an optical amplifier.

It is preferable that the at least one of the plurality of loopedoptical transmission lines is further connected to at least two set ofan optical receiver and an optical transmitter so that the opticalloop-structured circuit has a function of an optical add-dropmultiplexer.

It is preferable that at least one of the plurality of looped opticaltransmission lines has at least single set of an optical attenuator andan optical isolator so that the optical loop-structured circuit has afunction of an optical equalizer.

It is preferable that at least two of the plurality of looped opticaltransmission lines are connected to an opticalmultiplexer/demultiplexer, whilst a single looped optical transmissionline is separated by the at least two of the plurality of looped opticaltransmission lines from the optical multiplexer/demultiplexer, so thatoptical signals are individually transmitted along the plurality oflooped optical transmission lines, and wherein all of the plurality ofoptical transmission line junctions have the optical devices.

It is preferable that each of the plurality of looped opticaltransmission lines has at least a single set of an optical amplifier andan optical isolator so that the optical loop-structured circuit has afunction of an optical amplifier.

It is preferable that each of the plurality of looped opticaltransmission lines is further connected to at least two set of anoptical receiver an optical transmitter so that the opticalloop-structured circuit has a function of an optical add-dropmultiplexer.

It is preferable that each of the plurality of looped opticaltransmission lines has at least a single set of an optical attenuatorand an optical isolator so that the optical loop-structured circuit hasa function of an optical equalizer.

It is preferable that the optical device comprises an opticalmultiplexer/demultiplexer.

It is preferable that the optical device comprises an opticalmultiplexer.

It is preferable that the optical device comprises an opticaldemultiplexer.

The tenth present invention provides an optical loop-structured circuithaving at least a plurality of looped optical transmission lines havingat least a plurality of optical transmission line junctions from whichat least three optical transmission lines extend, wherein at least oneof the plurality of optical transmission line junctions has an opticalcirculator, which is connected to the at least three opticaltransmission lines, so that the optical circulator serves as a same rollas an optical coupler so as to reduce an optical power loss when theoptical signal is transmitted through the optical transmission linejunction structure.

It is preferable that all of the plurality of optical transmission linejunctions have the optical circulators.

It is preferable that at least one of the plurality of looped opticaltransmission lines has at least a single set of an optical amplifier andan optical isolator so that the optical loop-structured circuit has afunction of an optical amplifier.

It is preferable that at least one of the plurality of looped opticaltransmission lines is further connected to at least two set of anoptical receiver and an optical transmitter so that the opticalloop-structured circuit has a function of an optical add-dropmultiplexer.

It is preferable that at least one of the plurality of looped opticaltransmission lines has at least a single set of an optical attenuatorand an optical isolator so that the optical loop-structured circuit hasa function of an optical equalizer.

It is preferable that at least two of the plurality of looped opticaltransmission lines are connected to an opticalmultiplexer/demultiplexer, whilst a single looped optical transmissionline is separated by the at least two of the plurality of looped opticaltransmission lines from the optical multiplexer/demultiplexer, so thatoptical signals are individually transmitted along the plurality oflooped optical transmission lines, and wherein all of the plurality ofoptical transmission line junctions have the optical circulators.

It is preferable that each of the plurality of looped opticaltransmission lines has at least a single set of an optical amplifier andan optical isolator so that the optical loop-structured circuit has afunction of an optical amplifier.

It is preferable that each of the plurality of looped opticaltransmission lines is further connected to at least two set of anoptical receiver and an optical transmitter so that the opticalloop-structured circuit has a function of an optical add-dropmultiplexer.

It is preferable that each of the plurality of looped opticaltransmission lines has at least a single set of an optical attenuatorand an optical isolator so that the optical loop-structured circuit hasa function of an optical equalizer.

The eleventh present invention provides an optical gate switchcomprising the following elements. A main optical transmission line isprovided. First and second optical multiplexer/demultiplexers are alsoprovided on the main optical transmission line so that the first andsecond optical multiplexer/demultiplexers are separated from each other.The first and second optical multiplexer/demultiplexers are connectedwith first and second subordinate optical transmission linesrespectively. An impurity doped fiber is provided on the main opticaltransmission line and positioned between the first and second opticalmultiplexer/demultiplexers. An excitation light source is provided whichis connected through the first subordinate optical transmission line tothe first optical multiplexer/demultiplexer so that the excitation lightsource emits an excitation light which is transmitted through the firstsubordinate optical transmission line and the first opticalmultiplexer/demultiplexer to the impurity doped fiber. The secondoptical multiplexer/demultiplexer transmits the optical signal onto themain optical transmission line and also transmits a leaked part of theexcitation light onto the second subordinate optical transmission line.

It is preferable to further comprise an optical reflecting mirrorprovided on the second subordinate optical transmission line forreflecting the leaked part of the excitation light to the impurity dopedfiber.

It is preferable to further comprise a secondary excitation light sourceon the second subordinate optical transmission line.

Preferred Embodiments

First Embodiment

A first embodiment according to the present invention will be describedin detail with reference to FIG. 1 which is a diagram illustrative of afirst novel optical switch having a single input and two outputs. Theoptical switch has an input side coupler 21 which is connected to afirst optical transmission line 110 on which an optical input signal istransmitted and then inputted into the optical switch. The optical inputsignal has a wavelength of 1550 nanometers and an intensity of 0 dBm.The optical input signal is divided by the input side coupler 21 intotwo parts. The optical switch has second and third optical transmissionlines 120 and 121 which are connected to the input side coupler 21. Thetwo divided optical signals are then transmitted through the second andthird optical transmission lines 120 and 121 for output thereof. Thesecond optical transmission line 120 is connected to a first output sidecoupler 22. The third optical transmission line 121 is connected to asecond output side optical coupler 23. A first erbium doped fiber EDF11is provided on the second optical transmission line 120 between theinput side coupler 21 and the first output side coupler 22. A seconderbium doped fiber EDF12 is provided on the third optical transmissionline 120 between the input side coupler 21 and the second output sidecoupler 23. The first and second erbium doped fibers EDF11 and EDF12have a length of 50 meters. The first and second erbium doped fibersEDF11 and EDF12 may be replaced by rare earth doped fibers. The twodivided optical signals are transmitted through the first and seconderbium doped fibers EDF11 and EDF12 respectively. The optical switchfurther has an excitation light switch 41 which is connected through afirst excitation light transmission line 111 to the first output sidecoupler 22 as well as which is connected through a second excitationlight transmission line 112 to the second output side coupler 23. Theoptical switch further has an excitation light source 31 which isconnected to the excitation light switch 41. The excitation light source31 emits an excitation light with a wavelength of 1480 nanometers Theexcitation light switch 41 is operated to switch the excitation light toany one of the first and second excitation light transmission lines 111and 112 to supply any one of the first and second erbium doped fibersEDF11 and EDF12.

If the excitation light switch 41 is operated to switch to supply theexcitation light to the first erbium doped fiber EDF11, then the firsterbium doped first EDF11 is excited whereby the divided optical signalwith the wavelength of 1550 nanometers is transmitted through the firsterbium doped fiber EDF11 without any optical absorption and then theoptical signal with an intensity of 0 dBm is outputted from the secondoptical transmission line 120. Accurately, the majority part of theexcitation light emitted from the excitation light source 31 is switchedby the excitation light switch 41 to be fed through the first excitationlight transmission line 111 and the first output side optical coupler 22to the first erbium doped fiber EDF11. On the other hand, the minoritypart of the excitation light emitted from the excitation light source 31might be leaked through the excitation light switch 41 whereby a leakedexcitation light is then fed through the second output side opticalcoupler 23 to the second erbium doped fiber EDF12. However, the leakedexcitation light is incapable of exciting the second erbium doped fiberEDF11, for which reason the divided optical signal with the wavelengthof 1550 nanometers is absorbed into the second erbium doped fiber EDF11.As a result, an optical output signal from the third opticaltransmission line 121 has an intensity of −60 dBm or less. Theexcitation light switch 41 causes an insertion loss of 2 dB and acrosstalk of 20 dB which allow the optical switch to be free from anysubstantive insertion loss and a low or reduced crosstalk.

As a modification to the above first embodiment, the above excitationlight switch 41 may be replaced by a polymer optical switch.

If the polymer optical switch 41 is operated to switch to supply theexcitation light to the first erbium doped fiber EDF11, then the firsterbium doped fiber EDF11 is excited whereby the divided optical signalwith the wavelength of 1550 nanometers is transmitted through the firsterbium doped fiber EDF11 without any optical absorption and then theoptical signal with an intensity of 0 dBm is outputted from the secondoptical transmission line 120. Accurately, the majority part of theexcitation light emitted from the excitation light source 31 is switchedby the polymer optical switch 41 to be fed through the first excitationlight transmission line 111 and the first output side optical coupler 22to the first erbium doped fiber EDF11. On the other hand, the minoritypart of the excitation light emitted from the excitation light source 31might be leaked through the polymer optical switch 41 whereby a leakedexcitation light is then fed through the second output side opticalcoupler 23 to the second erbium doped fiber EDF12. However, the leakedexcitation light is incapable of exciting the second erbium doped fiberEDF12, for which reason the divided optical signal with the wavelengthof 1550 nanometers is absorbed into the second erbium doped fiber EDF12.As a result, an optical output signal from the third opticaltransmission line 121 has intensity of −60 dBm or less. The polymeroptical switch 41 causes an insertion loss of 2 dB and a crosstalk of 20dB which allow the optical switch to be free from any substantiveinsertion loss and a low or reduced crosstalk.

If the polymer optical switch 41 is operated to switch to supply theexcitation light to the second erbium doped fiber EDF12, then the seconderbium doped fiber EDF12 is excited whereby the divided optical signalwith the wavelength of 1550 nanometers is transmitted through the seconderbium doped fiber EDF12 without any optical absorption and then theoptical signal with an intensity of 0 dBm is outputted from the thirdoptical transmission line 121. Accurately, the majority part of theexcitation light emitted from the excitation light source 31 is switchedby the polymer optical switch 41 to be fed through the second excitationlight transmission line 112 and the second output side optical coupler23 to the second erbium doped fiber EDF12. On the other hand, theminority part of the excitation light emitted from the excitation lightsource 31 might be leaked through the polymer optical switch 41 wherebya leaked excitation light is then fed through the first output sideoptical coupler 22 to the first erbium doped fiber EDF11. However, theleaked excitation light is incapable of exciting the first erbium dopedfiber EDF11, for which reason the divided optical signal with thewavelength of 1550 nanometers is absorbed into the first erbium dopedfiber EDF11. As a result, an optical output signal from the thirdoptical transmission line 121 has an intensity of −60 dBm or less. Thepolymer optical switch 41 causes an insertion loss of 2 dB and acrosstalk of 20 dB which allow the optical switch to be free from anysubstantive insertion loss and a low or reduced crosstalk.

As a further modification to the above first embodiment, the excitationlight has a wavelength of 980 nanometers in order to shorten thewavelength for a remarkable reduction in noise factor of the opticaloutput signal. In this case, the optical switch is also free from anysubstantive insertion loss and a low or reduced crosstalk.

In the above embodiment, the number of the wavelength multiplexing oneach optical transmission line is one. Notwithstanding, 8, 16, 32,64-wavelength multiplexing are available, wherein the batch-switchingoperation to the plural number wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal atnot only 1550 nanometers but also other wavelengths, for example, 1330nanometers.

It is also possible to set the wavelength of the excitation light at notonly 1480 nanometers or 980 nanometers but also other wavelengthsprovided that such wavelength is capable of exciting the impurity dopedfiber. It is preferable to set the wavelength of the excitation light inconsideration of both the wavelength of the optical input signal and thekind of the impurity doped fiber.

The above excitation light switch may also be replaced by anacousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical outputsignal by controlling an optical power of the excitation light to be fedto the impurity doped fiber. It is possible to control the optical powerof the excitation light to be fed to the impurity doped fiber bycontrolling an injection current to the excitation light source or byuse of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rareearth doped fiber such as tellurium doped fiber. The length of the rareearth doped fiber and a doping concentration thereof may be set inaccordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rareearth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing todifferent excitation lights emitted separately from plural differentexcitation light sources in order to input the polarization-multiplexedexcitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical division at theoptical coupler in accordance with the various design choices.

The provisions of the smaller number of the excitation light source andthe single excitation light switch permit ON-OFF switching operations ofthe plural gate switches by a simple structure. The above switchexhibits such a gain property as a sharp rising, for which reason thereis substantially no influence due to a leaked light from the excitationlight switch. This makes the switch available to switches havingrelatively large crosstalk levels such as a polymer type switch orLiNbO.sub.3 switch, thereby realizing a low crosstalk and low insertionloss optical switch. In addition, the use of the impurity doped fiberserving as an optical power amplifier can obtain a gain as the opticalswitch.

Second Embodiment

A second embodiment according to the present invention will be describedin detail with reference to FIG. 2 which is a diagram illustrative of asecond novel optical switch having a single input and two outputs. Astructural difference of the second novel optical switch from the firstnovel optical switch is only in further providing first and secondoptical filters on two output sides in order to eliminate or removeamplified noises from the optical output signals.

The optical switch has an input side coupler 21 which is connected to afirst optical transmission line 110 on which an optical input signal istransmitted and then inputted into the optical switch. The optical inputsignal has a wavelength of 1550 nanometers and an intensity of 0 dBm.The optical input signal is divided by the input side coupler 21 intotwo parts. The optical switch has second and third optical transmissionlines 120 and 121 which are connected to the input side coupler 21. Thetwo divided optical signals are then transmitted through the second andthird optical transmission lines 120 and 121 for output thereof. Thesecond optical transmission line 120 is connected to a first output sidecoupler 22. The third optical transmission line 121 is connected to asecond output side optical coupler 23. A first erbium doped fiber EDF11is provided on the second optical transmission line 120 between theinput side coupler 21 and the fist output side coupler 22. A seconderbium doped fiber EDF12 is provided on the third optical transmissionline 120 between the input side coupler 21 and the second output sidecoupler 23. The first and second erbium doped fibers EDF11 and EDF12have a length of 50 meters. The first and second erbium doped fibersEDF11 and EDF12 may be replaced by rare earth doped fibers. The twodivided optical signals are transmitted through the first and seconderbium doped fibers EDF11 and EDF12 respectively. The optical switchfurther has an excitation light switch 41 which is connected through afirst excitation light transmission line 111 to the first output sidecoupler 22 as well as which is connected through a second excitationlight transmission line 112 to the second output side coupler 23. Theoptical switch further has an excitation light source 31 which isconnected to the excitation light switch 41. The excitation light source31 emits an excitation light with a wavelength of 1480 nanometers. Theexcitation light switch 41 is operated to switch the excitation light toany one of the first and second excitation light transmission lines 111and 112 to supply any one of the first and second erbium doped fibersEDF11 and EDF12.

Further, in this second embodiment, a first optical filter 51 isprovided on the second optical transmission line 120 and positionedcloser to the output side than the first output side optical coupler 22.If the first erbium doped fiber EDF11 is excited, then this first erbiumdoped fiber EDF11 also serves as an optical power amplifier which,however, amplifies not only the divided optical signal from the firstoptical transmission line 110 but also noises induced in the opticalsignals, for which reason it is preferable to remove or eliminate thenoises from the optical output signal by the first optical filter 51 inorder to avoid deterioration in signal-to-noise ratio due to provisionof the excitation light switch 41. Similarly, a second optical filter 52is provided on the third optical transmission line 121 and positionedcloser to the output side than the second output side optical coupler23. If the second erbium doped fiber EDF12 is excited, then this seconderbium doped fiber EDF12 also serves as an optical power amplifierwhich, however, amplifies not only the divided optical signal from thefirst optical transmission line 110 but also noises included in theoptical signals, for which reason it is preferable to remove oreliminate the noises from the optical output signal by the secondoptical filter 52 in order to avoid deterioration in signal-to-noiseratio due to provision of the excitation light switch 41.

If the excitation light switch 41 is operated to switch to supply theexcitation light to the first erbium doped fiber EDF11, then the firsterbium doped fiber EDF11 is excited whereby the divided optical signalwith the wavelength of 1550 nanometers is transmitted through the firsterbium doped fiber EDF11 without any optical absorption. The opticaloutput signal is then fed to the first optical filter 51 to remove oreliminate the noises from the optical output signal by the first opticalfilter 51 in order to avoid deterioration in signal-to-noise ratio dueto provision of the excitation light switch 41. Therefore, the opticalsignal filtered in wavelength and having an intensity of 0 dBm isoutputted from the second optical transmission line 120. Accurately, themajority part of the excitation light emitted from the excitation lightsource 31 is switched by the excitation light switch 41 to be fedthrough the first excitation light transmission line 111 and the firstoutput side optical coupler 22 to the first erbium doped fiber EDF11. Onthe other hand, the minority part of the excitation light emitted fromthe excitation light source 31 might be leaked through the excitationlight switch 41 whereby a leaked excitation light is then fed throughthe second output side optical coupler 23 to the second erbium dopedfiber EDF12. However, the leaked excitation light is incapable ofexciting the second erbium doped fiber EDF12, for which reason thedivided optical signal with the wavelength of 1550 nanometers isabsorbed into the second erbium doped fiber EDF12. As a result, anoptical output signal from the third optical transmission line 121 isfree of any substantive noise and has an intensity of −60 dBm or less.The excitation light switch 41 causes an insertion loss of 2 dB and acrosstalk of 20 dB which allow the optical switch to be free from anysubstantive insertion loss and a low or reduced crosstalk.

If the excitation light switch 41 is operated to switch to supply theexcitation light to the second erbium doped fiber EDF12, then the seconderbium doped fiber EDF12 is excited whereby the divided optical signalwith the wavelength of 1550 nanometers is transmitted through the seconderbium doped fiber EDF12 without any optical absorption. The opticaloutput signal is then fed to the second optical filter 52 to remove oreliminate the noises from the optical output signal by the secondoptical filter 52 in order to avoid deterioration in signal-to-noiseratio due to provision of the excitation light switch 41. Therefore, theoptical signal filtered in wavelength and having an intensity of 0 dBmis outputted from the second optical transmission line 120. Accurately,the majority part of the excitation light emitted from the excitationlight source 31 is switched by the excitation light switch 41 to be fedthrough the second excitation light transmission line 112 and the secondoutput side optical coupler 23 to the second erbium doped fiber EDF12.On the other hand, the minority part of the excitation light emittedfrom the excitation light source 31 might be leaked through theexcitation light switch 41 whereby a leaked excitation light is then fedthrough the first output side optical coupler 22 to the first erbiumdoped fiber EDF11. However, the leaked excitation light is incapable ofexciting the first erbium doped fiber EDF11, for which reason thedivided optical signal with the wavelength of 1550 nanometers isabsorbed into the first erbium doped fiber EDF11. As a result, anoptical output signal from the third optical transmission line 121 isfree of any substantive noise and has an intensity of −60 dBm or less.The excitation light switch 41 causes an insertion loss of 2 dBm and acrosstalk of 20 dB which allow the optical switch to be free from anysubstantive insertion loss and a low or reduced crosstalk.

As a modification to the above second embodiment, the above excitationlight switch 41 may be replaced by a polymer optical switch similarly tothe first embodiment.

If the polymer optical switch 41 is operated to switch to supply theexcitation light to the first erbium doped fiber EDF11, then the firsterbium doped fiber EDF11 is excited whereby the divided optical signalwith the wavelength of 1550 nanoseconds is transmitted through the firsterbium doped fiber EDF11 without any optical absorption. The opticaloutput signal is then fed to the first optical filter 51 to remove oreliminate the noises from the optical output signal by the first opticalfilter 51 in order to avoid deterioration in signal-to-noise ratio dueto provision of the polymer optical switch 41. Therefore, the opticalsignal filtered in wavelength and having an intensity of 0 dBm isoutputted from the second optical transmission line 120. Accurately, themajority part of the excitation light emitted from the excitation lightsource 31 is switched by the polymer optical switch 41 to be fed throughthe first excitation light transmission line 111 and the first outputside optical coupler 22 to the first erbium doped EDF11. On the otherhand, the minority part of the excitation light emitted from theexcitation light source 31 might be leaked through the polymer opticalswitch 41 whereby a leaked excitation light is then fed through thesecond output side optical coupler 23 to the second erbium doped fiberEDF12. However, the leaked excitation light is incapable of exciting thesecond erbium doped fiber EDF12, for which reason the divided opticalsignal with the wavelength of 1550 nanometers is absorbed into thesecond erbium doped fiber EDF12. As a result, an optical output signalfrom the third optical transmission line 121 is free of any substantivenoise and has an intensity of −60 dBm or less. The polymer opticalswitch 41 causes an insertion loss of 20 dB and a crosstalk of 20 dBwhich allow the optical switch to be free from any substantive insertionloss and a low or reduced crosstalk.

If the polymer optical switch 41 is operated to switch to supply theexcitation light to the second erbium doped fiber EDF12, then the seconderbium doped fiber EDF12 is excited whereby the divided optical signalwith the wavelength of 1550 nanometers is transmitted through the seconderbium doped fiber EDF12 without any optical absorption. The opticaloutput signal is then fed to the second optical filter 52 to remove oreliminate the noises from the optical output signal by the secondoptical filter 52 in order to avoid deterioration in signal-to-noiseratio due to provision of the polymer optical switch 41. Therefore, theoptical signal filtered in wavelength and having an intensity of 0 dBmis outputted from the second optical transmission line 120. Accurately,the majority part of the excitation light emitted from the excitationlight source 31 is switched by the polymer optical switch 41 to be fedthrough the second excitation light transmission line 112 and the secondoutput side coupler 23 to the second erbium doped fiber EDF12. On theother hand, the minority part of the excitation light emitted from theexcitation light source 31 might be leaked through the polymer opticalswitch 41 whereby a leaked excitation light is then fed through thefirst output side optical coupler 22 to the first erbium doped fiberEDF11. However, the leaked excitation light is incapable of exciting thefirst erbium doped fiber EDF11, for which reason the divided opticalsignal with the wavelength of 1550 nanometers is absorbed into the firsterbium doped fiber EDF11. As a result, an optical output signal from thethird optical transmission line 121 is free of any substantive noise andhas no intensity of −60 dBm or less. The polymer optical switch 41causes an insertion loss of 2 dB and a crosstalk of 20 dB which allowthe optical switch to be free from any substantive insertion loss and alow or reduced crosstalk.

As a further modification to the above second embodiment, the excitationlight has a wavelength of 980 nanometers in order to shorten thewavelength for a remarkable reduction in noise factor of the opticaloutput signal. In this case, the optical switch is also free from anysubstantive insertion loss and a low or reduced crosstalk.

In the above embodiment, the number of the wavelength multiplexing oneach optical transmission line is one. Notwithstanding, 8, 16, 32,64-wavelength multiplexing are available, wherein the batch-switchingoperation to the plural number wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal atnot only 1550 nanometers but also other wavelengths, for example, 1330nanometers.

It is also possible to set the wavelength of the excitation light at notonly 1480 nanometers or 980 nanometers but also other wavelengthsprovided that such wavelength is capable of exciting the impurity dopedfiber. It is preferable to set the wavelength of the excitation light inconsideration of both the wavelength of the optical input signal and thekind of the impurity doped fiber.

The above excitation light switch may also be replaced by anacousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical outputsignal by controlling an optical power of the excitation light to be fedto the impurity doped fiber. It is possible to control the optical powerof the excitation light to be fed to the impurity doped fiber bycontrolling an injection current to the excitation light source or byuse of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rearearth doped fiber such as tellurium doped fiber. The length of the rareearth doped fiber and a doping concentration thereof may be set inaccordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rareearth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing todifferent excitation lights emitted separated from plural differentexcitation light sources in order to input the polarization-multiplexedexcitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical devices at theoptical coupler in accordance with the various design choices.

It is still further possible to freely set the transmission-band widthin accordance with the number of the optical signals to be transmittedthrough the optical switch.

It is yet further possible to provide optical filters and opticalisolators since the excitation light and returned light provide noinfluence to input and output sides of the optical switch.

The provisions of the smaller number of the excitation light source andthe single excitation light switch permit ON-OFF switching operations ofthe plural gate switches by a simple structure. The above switchexhibits such a gain property as a sharp rising, for which reason thereis substantially no influence due to a leaked light from the excitationlight switch. This makes the switch available to switches havingrelatively large crosstalk levels such as a polymer type switch orLiNbO.sub.3 switch, thereby realizing a low crosstalk and low insertionloss optical switch. In addition, the use of the impurity doped fiberserving as an optical power amplifier can obtain a gain as the opticalswitch.

Third Embodiment

A third embodiment according to the present invention will be describedin detail with reference to FIG. 3 which is a diagram illustrative of athird novel optical switch having a single input and two outputs. Astructural difference of the third novel optical switch from the firstnovel optical switch is in further providing first and second opticalisolators as well as first and second optical mirrors in order toincrease an efficiency of excitation of the erbium doped fiber withallowance of a sufficient optical absorption.

The optical switch has an input side coupler 21 which is connected to afirst optical transmission line 110 on which an optical switch signal istransmitted and then inputted into the optical switch. The optical inputsignal has a wavelength of 1550 nanometers and an intensity of 0 dBm.The optical input signal is divided by the input side coupler 21 intotwo parts. The optical switch has second and third optical transmissionlines 120 and 121 which are connected to the input side coupler 21. Thetwo divided optical signals are then transmitted through the second andthird optical transmission lines 120 and 121 for output thereof. Thesecond optical transmission line 120 is connected to a first output sidecoupler 22. The third optical transmission line 121 is connected to asecond output side optical coupler 23. A first erbium doped fiber EDF11is provided on the second optical transmission line 120 between theinput side coupler 21 and the first output side coupler 22. A seconderbium doped fiber EDF12 is provided on the third optical transmissionline 120 between the input side coupler 21 and the second output sidecoupler 23. The first and second erbium doped fibers EDF11 and EDF12have a length of 50 meters. The first and second erbium doped fibersEDF11 and EDF12 may be replaced by rare earth doped fibers. The twodivided optical signals are transmitted through the first and seconderbium doped fibers EDF11 and EDF12 respectively. The optical switchfurther has an excitation light switch 41 which is connected through afirst excitation light transmission line 111 to the first output sidecoupler 22 as well as which is connected through a second excitationlight transmission line 112 to the second output side coupler 23. Theoptical switch further has an excitation light source 31 which isconnected to the excitation light switch 41. The excitation light source31 emits an excitation light with a wavelength of 1480 nanometers. Theexcitation light switch 41 is operated to switch the excitation light toany one of the first and second excitation light transmission line 111and 112 to supply any one of the first and second erbium doped fibersEDF11 and EDF12.

In addition, a first optical isolator 61 is provided on the secondoptical transmission line 120 and positioned between the input sideoptical coupler 21 and the first erbium doped fiber EDF11. The firstoptical isolator 61 permits only a unidirectional transmission of theoptical signal from the input side optical coupler 21 to the firsterbium doped fiber EDF11, however, preventing an opposite directiontransmission of the optical signal from the first erbium doped fiberEDF11 to the input side optical coupler 21. A second optical isolator 62is provided on the third optical transmission line 121 and positionedbetween the input side optical coupler 21 and the second erbium dopedfiber EDF12. The second optical isolator 62 permits only aunidirectional transmission of the optical signal from the input sideoptical coupler 21 to the second erbium doped fiber EDF12, however,preventing an opposite direction transmission of the optical signal fromthe second erbium doped fiber EDF12 to the input side optical coupler21. Moreover, a first optical reflective mirror 71 is provided on afirst terminal of the second optical transmission line 120 so that thedivided optical signal having passed through the first erbium dopedfiber EDF11 is reflected by the first optical reflective mirror 71toward the first erbium doped fiber EDF11, whereby the divided opticalsignal passes through the first erbium doped fiber EDF11 two times. Ifthe first erbium doped fiber EDF11 is excited, then this first erbiumdoped fiber EDF11 serves as an amplifier. This two times transmissionsof the divided optical signal by the first optical reflective mirror 71increases the efficiency of the excitation of the first erbium dopedfiber EDF11 even if the power of the excitation light emitted from theexcitation light source 31 is not so high. The reflected optical signalis thus transmitted through the first erbium doped fiber EDF11 anddivided into two parts, wherein one of the further divided parts of thereflected optical signal is outputted from an output terminal of afourth optical transmission line 122 whilst transmission of theremaining one of the further divided parts of the reflected opticalsignal is discontinued by the first optical isolator 61 so that no lightis transmitted back to the first optical transmission line 110.Furthermore, a second optical reflective mirror 72 is provided on asecond terminal of the third optical transmission line 121 so that thedivided optical signal having passed through the second erbium dopedfiber EDF12 is reflected by the second optical reflective mirror 72toward the second erbium doped fiber EDF12, whereby the divided opticalsignal passes through the second erbium doped fiber EDF12 two times. Ifthe second erbium doped fiber EDF12 is excited, then this second erbiumdoped fiber EDF12 serves as an amplifier. This two times transmissionsof the divided optical signal by the second optical reflective mirror 72increases the efficiency of the excitation of the second erbium dopedfiber EDF12 even if the power of the excitation light emitted from theexcitation light source 31 is not so high. The reflected optical signalis thus transmitted through the second erbium doped fiber EDF12 anddivided into two parts, wherein one of the further divided parts of thereflected optical signal is outputted from an output terminal of a fifthoptical transmission line 123 whilst transmission of the remaining oneof the further divided parts of the reflected optical signal isdiscontinued by the second optical isolator 62 so that no light istransmitted back to the first optical transmission line 110.

If the excitation light switch 41 is operated to switch to supply theexcitation light to the first erbium doped fiber EDF11, then the firsterbium doped fiber EDF11 is excited whereby the divided optical signalwith the wavelength of 1550 nanometers is transmitted through the firsterbium doped fiber EDF11 without any optical absorption and then theoptical signal is reflected by the first optical reflective mirror 71for subsequent returning to the first erbium doped fiber EDF11. This twotimes transmissions of the divided optical signal by the first opticalreflective mirror 71 increases the efficiency of the excitation of thefirst erbium doped fiber EDF11, even if the power of the excitationlight emitted from the excitation light source 31 is not so high. Thereflected optical signal is thus transmitted through the first erbiumdoped fiber EDF11 and divided by an optical coupler into two parts,wherein one of the further divided parts of the reflected optical signalis outputted from an output terminal of a fourth optical transmissionline 122 whilst transmission of the remaining one of the further dividedparts of the reflected optical signal is discontinued by the firstoptical isolator 61 so that no light is transmitted back to the firstoptical transmission line 110. The optical signal with an intensity of 0dBm is outputted from the output terminal of the fourth opticaltransmission line 122. Accurately, the majority part of the excitationlight emitted from the excitation light source 31 is switched by theexcitation light switch 41 to be fed through the first excitation lighttransmission line 111 and the first output side optical coupler 22 tothe first erbium doped fiber EDF11. On the other hand, the minority partof the excitation light emitted from the excitation light source 31might be leaked through the excitation light switch 41 whereby a leakedexcitation light is then fed through the second output side opticalcoupler 23 to the second erbium doped fiber EDF12. However, the leakedexcitation light is incapable of exciting the second erbium doped fiberEDF12, for which reason the divided optical signal with the wavelengthof 1550 nanometers in absorbed into the second erbium doped fiber EDF12.A leaked divided optical signal is also reflected by the second opticalreflective mirror 72 and the reflected leaked optical signal is againtransmitted through the second erbium doped fiber EDF12. As a result, anoptical output signal from the fifth optical transmission line 123 hasan intensity of −80 dBm or less. The excitation light switch 41 causesan insertion loss of 2 dB and a crosstalk of 20 dB which allow theoptical switch to be free from any substantive insertion loss and a lowor reduced crosstalk.

If the excitation light switch 41 is operated to switch to supply theexcitation light to the second erbium doped fiber EDF12, then the seconderbium doped fiber EDF12 is excited whereby the divided optical signalwith the wavelength of 1550 nanometers is transmitted through the seconderbium doped fiber EDF12 without any optical absorption and then theoptical signal is reflected by the second optical reflective mirror 72for subsequent returning to the second erbium doped fiber EDF12. Thistwo times transmissions of the divided optical signal by the secondoptical reflective mirror 72 increases the efficiency of the excitationof the second erbium doped fiber EDF12 even if the power of theexcitation light emitted from the excitation light source 31 is not sohigh. The reflected optical signal is thus transmitted through thesecond erbium doped fiber EDF12 and divided by an optical coupler intotwo parts, wherein one of the further divided parts of the reflectedoptical signal is outputted from an output terminal of a fifth opticaltransmission line 123 whilst transmission of the remaining one of thefurther divided parts of the reflected optical signal is discontinued bythe second optical isolator 62 so that no light is transmitted back tothe first optical transmission line 110. The optical signal with anintensity of 0 dBm is outputted from the output terminal of the fourthoptical transmission line 122. Accurately, the majority part of theexcitation light emitted from the excitation light source 31 is switchedby the excitation light switch 41 to be fed through the secondexcitation light transmission line 112 and the second output sideoptical coupler 23 to the second erbium doped fiber EDF12. On the otherhand, the minority part of the excitation light emitted from theexcitation light source 31 might be leaked through the excitation lightswitch 41 whereby a leaked excitation light is then fed through thefirst output side optical coupler 22 to the first erbium doped fiberEDF11. However, the leaked excitation light is incapable of exciting thefirst erbium doped fiber EDF11, for which reason the divided opticalsignal with the wavelength of 1550 nanometers is absorbed into the firsterbium doped fiber EDF11. A leaked divided optical signal is alsoreflected by the first optical reflective mirror 71 and the reflectedleaked optical signal is again transmitted through the first erbiumdoped fiber EDF11. As a result, an optical output signal from the fourthoptical transmission line 122 has an intensity of −80 dBm or less. Theexcitation light switch 41 causes an insertion loss of 2 dBm and acrosstalk of 20 dB which allow the optical switch to be free from anysubstantive insertion loss and a low or reduced crosstalk.

As a modification to the above third embodiment, the above excitationlight switch 41 may be replaced by a polymer optical switch.

If the polymer optical switch 41 is operated to switch to supply theexcitation light to the first erbium doped fiber EDF11, then the firsterbium doped fiber EDF11 is excited whereby the divided optical signalwith the wavelength of 1550 nanometers is transmitted through the firsterbium doped fiber EDF11 without any optical absorption and then theoptical signal is reflected by the first optical reflective mirror 71for subsequent returning to the first erbium doped fiber EDF11. This twotimes transmissions of the divided optical signal by the first opticalreflective mirror 71 increases the efficiency of the excitation of thefirst erbium doped fiber EDF11 even if the power of the excitation lightemitted from the excitation light source 31 is not so high. Thereflected optical signal is thus transmitted through the first erbiumdoped fiber EDF11 and divided by an optical coupler into two parts,wherein one of the further divided parts of the reflected optical signalis outputted from an output terminal of a fourth optical transmissionline 122 whilst transmission of the remaining one of the further dividedparts of the reflected optical signal is discontinued by the firstoptical isolator 61 so that no light is transmitted back to the firstoptical transmission line 110. The optical signal with an intensity of 0dBm is outputted from the output terminal of the fourth opticaltransmission line 122. Accurately, the majority part of the excitationlight emitted from the excitation light source 31 is switched by thepolymer optical switch 41 to be fed through the first excitation lighttransmission line 111 and the first output side optical coupler 22 tothe first erbium doped fiber EDF11. On the other hand, the minority partof the excitation light emitted from the excitation light source 31might be leaked through the polymer optical switch 41 whereby a leakedexcitation light is then fed through the second output side opticalcoupler 23 to the second erbium doped fiber EDF12. However, the leakedexcitation light is incapable of exciting the second erbium doped fiberEDF12, for which reason the divided optical signal with the wavelengthof 1550 nanometers is absorbed into the second erbium doped fiber EDF12.A leaked divided optical signal is also reflected by the second opticalreflective mirror 72 and the reflected leaked optical signal is againtransmitted through the second erbium doped fiber EDF12. As a result, anoptical output signal from the fifth optical transmission line 123 hasan intensity of −80 dBm or less. The polymer optical switch 41 causes aninsertion loss of 2 dB and a crosstalk of 20 dB which allow the opticalswitch to be free from any substantive insertion loss and a low orreduced crosstalk.

If the polymer optical switch 41 is operated to switch to supply theexcitation light to the second erbium doped fiber EDF12, then the seconderbium doped fiber EDF12 is excited whereby the divided optical signalwith the wavelength of 1550 nanometers is transmitted through the seconderbium doped fiber EDF12 without any optical absorption and then theoptical signal is reflected by the second optical reflective mirror 72for subsequent returning to the second erbium doped fiber EDF12. Thistwo times transmissions of the divided optical signal by the secondoptical reflective mirror 72 increases the efficiency of the excitationof the second erbium doped fiber EDF12 even if the power of theexcitation light emitted from the excitation light source 31 is not sohigh. The reflected optical signal is thus transmitted through thesecond erbium doped fiber EDF12 and divided by an optical coupler intotwo parts, wherein one of the further divided parts of the reflectedoptical signal is outputted from an output terminal of a fifth opticaltransmission line 123 whilst transmission of the remaining one of thefurther divided parts of the reflected optical signal is discontinued bythe second optical isolators 62 so that no light is transmitted back tothe first optical transmission line 110. The optical signal with anintensity of 0 dBm is outputted from the output terminal of the fourthoptical transmission line 122. Accurately, the majority part of theexcitation light emitted from the excitation light source 31 is switchedby the polymer optical switch 41 to be fed through the second excitationlight transmission line 112 and the second output side optical coupler23 to the second erbium doped fiber EDF12. On the other hand, theminority part of the excitation light emitted from the excitation lightsource 31 might be leaked through the polymer optical switch 41 wherebya leaked excitation light is then fed through the first output sideoptical coupler 22 to the first erbium doped fiber EDF11. However, theleaked excitation light is incapable of exciting the first erbium dopedfiber EDF11, for which reason the divided optical signal with thewavelength of 1550 nanometers is absorbed into the first erbium dopedfiber EDF11. A leaked divided optical signal is also reflected by thefirst optical reflective mirror 71 and the reflected leaked opticalsignal is again transmitted through the first erbium doped fiber EDF11.As a result, an optical signal from the fourth optical transmission line122 has an intensity of −80 dBm or less. The polymer optical switch 41causes an insertion loss of 2 dBm and a crosstalk of 20 dB which allowthe optical switch to be free from any substantive insertion loss and alow or reduced crosstalk.

As a further modification to the above third embodiment, the excitationlight has a wavelength of 980 nanometers in order to shorten thewavelength for a remarkable reduction in noise factor of the opticaloutput signal. In this case, the optical switch is also free from anysubstantive insertion loss and a low or reduced crosstalk.

In the above embodiment, the number of the wavelength multiplexing oneach optical transmission line is one. Notwithstanding, 8, 16, 32,64-wavelength multiplexing are available, wherein the batch-switchingoperation to the plural number wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal atnot only 1550 nanometers but also other wavelengths, for example, 1330nanometers.

It is also possible to set the wavelength of the excitation light at notonly 1480 nanometers or 980 nanometers but also other wavelengthsprovided that such wavelength is capable of exciting the impurity dopedfiber. It is preferable to set the wavelength of the excitation light inconsideration of both the wavelength of the optical input signal and thekind of the impurity doped fiber.

The above excitation light switch may also be replaced by anacousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical outputsignal by controlling an optical power of the excitation light to be fedto the impurity doped fiber. It is possible to control the optical powerof the excitation light to be fed to the impurity doped fiber bycontrolling an injection current to the excitation light source or byuse of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rareearth doped fiber such as tellurium doped fiber. The length of the rareearth doped fiber and a doping concentration thereof may be set inaccordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rareearth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing todifferent excitation lights emitted separately from plural differentexcitation light sources in order to input the polarization-multiplexedexcitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical division at theoptical coupler in accordance with the various design choices.

It is still further possible that the optical input and outputtransmission lines are used commonly or separately according to therequired optical system.

It is yet further possible to replace the input side coupler 21 and thefirst and second optical isolators 61 and 62 by a circulator.

It is additionally possible to provide optical reflective mirrors havingfixed or variable reflectivity as the first and second opticalreflective mirrors 71 and 72. If the variable reflectivity type opticalreflective mirrors are provided, it is possible to control the opticalpowers of the output signals.

The provisions of the smaller number of the excitation light source andthe single excitation light switch permit ON-OFF switching operations ofthe plural gate switches by a simple stricture. The above switchexhibits such a gain property as a sharp rising, for which reason thereis substantially no influence due to a leaked light from the excitationlight switch. This makes the switch available to switches havingrelatively large crosstalk levels such as a polymer type switch orLiNbO.sub.3 switch, thereby realizing a low crosstalk and low insertionloss optical switch. In addition, the use of the impurity doped fiberserving as an optical power amplifier can obtain a gain as the opticalswitch.

Fourth Embodiment

A fourth embodiment according to the present invention will be describedin detail with reference to FIG. 4 which is a diagram illustrative of afourth novel optical switch having a single input and two outputs. Astructural difference of the fourth novel optical switch from the firstnovel optical switch is in positions of first and second erbium dopedfibers so that first and second erbium doped fibers receive anexcitation light in the same direction as receipt of the opticalsignals, whilst in the first embodiment the first and second erbiumdoped fibers receive the excitation light in the opposite direction tothe receipt of the optical signals.

The optical switch has an input side coupler 21 which is connected to afirst optical transmission line 110 on which an optical input signal istransmitted and then inputted into the optical switch. The optical inputsignal has a wavelength of 1550 nanometers and an intensity of 0 dBm.The optical light signal is divided by the input side coupler 21 intotwo parts. The optical switch has second and third optical transmissionlines 120 and 121 which are connected to the input side coupler 21. Thetwo divided optical signals are then transmitted through the second andthird optical transmission lines 120 and 121 for output thereof. Thesecond optical transmission line 120 is connected to a first output sidecoupler 22. The third optical transmission line 121 is connected to asecond output side optical coupler 23. A first erbium doped fiber EDF11is provided on the second optical transmission line 120 and positionedbetween the first output side coupler 22 and the output terminal of thesecond optical transmission line 120. A second erbium doped fiber EDF12is provided on the third optical transmission line 120 and positionedbetween the second output side coupler 23 and the output terminal of thethird optical transmission line 121. The first and second erbium dopedfibers EDF11 and EDF12 have a length of 50 meters. The first and seconderbium doped fibers EDF11 and EDF12 may be replaced by rare earth dopedfibers. The two divided optical signals are transmitted through thefirst and second erbium doped fibers EDF11 and EDF12 respectively. Theoptical switch further has an excitation light switch 41 which isconnected through a first excitation light transmission line 111 to thefirst output side coupler 22 as well as which is connected through asecond excitation light transmission line 112 to the second output sidecoupler 23. The optical switch further has an excitation light source 31which is connected to the excitation light switch 41. The excitationlight source 31 emits an excitation light with a wavelength of 1480nanometers. The excitation light switch 41 is operated to switch theexcitation light to any one of the first and second excitation lighttransmission lines 111 and 112 to supply any one of the first and seconderbium doped fibers EDF11 and EDF12, so that the selected one of thefirst and second erbium doped fibers EDF11 and EDF12 receives theexcitation light in the same direction as receipt of the optical signal.

If the excitation light switch 41 is operated to switch to supply theexcitation light to the first erbium doped fiber EDF11 so that the firstand second erbium doped fiber EDF11 receives the excitation light in thesame direction as receipt of the optical signal, then the first erbiumdoped fiber EDF11 is excited whereby the divided optical signal with thewavelength of 1550 nanometers is transmitted through the first erbiumdoped fiber EDF11 without any optical absorption and then the opticalsignal with an intensity of 0 dBm is outputted from the second opticaltransmission line 120. Accurately, the majority part of the excitationlight emitted from the excitation light source 31 is switched by theexcitation light switch 41 to be fed through the first excitation lighttransmission line 111 and the first output side optical coupler 22 tothe first erbium doped fiber EDF11. On the other hand, the minority partof the excitation light emitted from the excitation light source 31might be leaked through the excitation light switch 41 whereby a leakedexcitation light is then fed through the second output side opticalcoupler 23 to the second erbium doped fiber EDF12. However, the leakedexcitation light is incapable of exciting the second erbium doped fiberEDF12, for which reason the divided optical signal with the wavelengthof 1550 nanometers is absorbed into the second erbium doped fiber EDF12.As a result, an optical output signal from the third opticaltransmission line 121 has an intensity of −60 dBm or less. Theexcitation light switch 41 causes an insertion loss of 2 dB and acrosstalk of 20 dB which allow the optical switch to be free from anysubstantive insertion loss and a low or reduced crosstalk.

If the excitation light switch 41 is operated to switch to supply theexcitation light to the second erbium doped fiber EDF12 so that thesecond erbium doped fiber EDF12 receives the excitation light in thesame direction as receipt of the optical signal, then the second erbiumdoped fiber EDF12 is excited whereby the divided optical signal with thewavelength of 1550 nanometers is transmitted through the second erbiumdoped fiber EDF12 without any optical absorption and then the opticalsignal with an intensity of 0 dBm is outputted from the third opticaltransmission line 121. Accurately, the majority part of the excitationlight emitted from the excitation light source 31 is switched by theexcitation light switch 41 to be fed through the second excitation lighttransmission line 112 and the second output side optical coupler 23 tothe second erbium doped fiber EDF12. On the other hand, the minoritypart of the excitation light emitted from the excitation light source 31might be leaked through the excitation light switch 41 whereby a leakedexcitation light is then fed through the first output side opticalcoupler 22 to the first erbium doped fiber EDF11. However, the leakedexcitation light is incapable of exciting the first erbium doped fiberEDF11, for which reason the divided optical signal with the wavelengthof 1550 nanometers is absorbed into the first erbium doped fiber EDF11.As a result, an optical output signal from the third opticaltransmission line 121 has an intensity of −60 dBm or less. Theexcitation light switch 41 causes an insertion loss of 2 dB and acrosstalk of 20 dB which allow the optical switch to be free from anysubstantive insertion loss and a low or reduced crosstalk.

As a modification to the above first embodiment, the above excitationlight switch 41 may be replaced by a polymer optical switch.

If the polymer optical switch 41 is operated to switch to supply theexcitation light to the first erbium doped fiber EDF11 so that the firstand second erbium doped fiber EDF11 receives the excitation light in thesame direction as receipt of the optical signal, then the first erbiumdoped fiber EDF11 is excited whereby the divided optical signal with thewavelength of 1550 nanometers is transmitted through the first erbiumdoped fiber EDF11 without any optical absorption and then the opticalsignal with an intensity of 0 dBm is outputted from the second opticaltransmission line 120. Accurately, the majority part of the excitationlight emitted from the excitation light source 31 is switched by thepolymer optical switch 41 to be fed through the first excitation lighttransmission line 111 and the first output side optical coupler 22 tothe first erbium doped fiber EDF11. On the other hand, the minority partof the excitation light emitted from the excitation light source 31might be leaked through the polymer optical switch 41 whereby a leakedexcitation light is then fed through the second output side opticalcoupler 23 to the second erbium doped fiber EDF12. However, the leakedexcitation light is incapable of exciting the second erbium doped fiberEDF12, for which reason the divided optical signal with the wavelengthof 1550 nanometers is absorbed into the second erbium doped fiber EDF12.As a result, an optical output signal from the third opticaltransmission line 121 has an intensity of −60 dBm or less. The polymeroptical switch 41 causes an insertion loss of 2 dB and a crosstalk of 20dB which allow the optical switch to be free from any substantiveinsertion loss and a low or reduced crosstalk.

If the polymer optical switch 41 is operated to switch to supply theexcitation light to the second erbium doped fiber EDF12 so that thesecond erbium doped fiber EDF12 receives the excitation light in thesame direction as receipt of the optical signal, then the second erbiumdoped fiber EDF12 is excited hereby the divided optical signal with thewavelength of 1550 nanometers is transmitted through the second erbiumdoped fiber EDF12 without any optical absorption and then the opticalsignal with an intensity of 0 dBm is outputted from the third opticaltransmission line 121. Accurately, the majority part of the excitationlight emitted from the excitation light source 31 is switched by thepolymer optical switch 41 to be fed through the second excitation lighttransmission line 112 and the second output side optical coupler 23 tothe second erbium doped fiber EDF12. On the other hand, the minoritypart of the excitation light emitted from the excitation light source 31might be leaked through the polymer optical switch 41 whereby a leakedexcitation light is then fed through the first output side opticalcoupler 22 to the first erbium doped fiber EDF11. However, the leakedexcitation light is incapable of exciting the first erbium doped fiberEDF11, for which reason the divided optical signal with the wavelengthof 1550 nanometers is absorbed into the first erbium doped fiber EDF11.As a result, an optical output signal from the third opticaltransmission line 121 has an intensity of −60 dBm or less. The polymeroptical switch 41 causes an insertion loss of 2 dB and a crosstalk of 20dB which allow the optical switch to be free from any substantiveinsertion loss and a low or reduced crosstalk.

As a further modification to the above first embodiment, the excitationlight has a wavelength of 980 nanometers in order to shorten thewavelength for a remarkable reduction in noise factor of the opticaloutput signal. In this case, the optical switch is also free from anysubstantive insertion loss and a low or reduced crosstalk.

In the above embodiment, the number of the wavelength multiplexing oneach optical transmission line is one. Notwithstanding, 8, 16, 32,64-wavelength multiplexing are available, wherein the batch-switchingoperation to the plural number wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal atnot only 1550 nanometers but also other wavelengths, for example, 1330nanometers.

It is also possible to set the wavelength of the excitation light at notonly 1480 nanometers or 980 nanometers but also other wavelengthsprovided that such a wavelength is capable of exciting the impuritydoped fiber. It is preferable to set the wavelength of the excitationlight in consideration of both the wavelength of the optical inputsignal and the kind of the impurity doped fiber.

The above excitation light switch may also be replaced by anacousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical outputsignal by controlling an optical power of the excitation light to be fedto the impurity doped fiber. It is possible to control the optical powerof the excitation light to be fed to the impurity doped fiber bycontrolling an injection current to the excitation light source or byuse of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rareearth doped fiber such as tellurium doped fiber. The length of the rareearth doped fiber and a doping concentration thereof may be set inaccordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rareearth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing todifferent excitation lights emitted separately from plural differentexcitation light sources in order to input the polarization-multiplexedexcitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical division at theoptical coupler in accordance with the various design choices.

The provisions of the smaller number of the excitation light source andthe single excitation light switch permit ON-OFF switching operations ofthe plural gate switches by a simple structure. The above switchexhibits such a gain property as a sharp rising, for which reason thereis substantially no influence due to a leaked light from the excitationlight switch. This makes the switch available to switches havingrelatively large crosstalk levels such as a polymer type switch orLiNbO.sub.3 switch, thereby realizing a low crosstalk and low insertionloss optical switch. In addition, the use of the impurity doped fiberserving as an optical power amplifier can obtain a gain as the opticalswitch.

Fifth Embodiment

A fifth embodiment according to the present invention will be describedin detail with reference to FIG. 5 which is a diagram illustrative of afifth novel optical switch having two inputs and two outputs. The fifthnovel optical switch comprises a pair of the above first novel opticalswitches described in the first embodiment. The two first novel opticalswitches are connected to each other through a first opticaltransmission line 110 as a common line. If the left side one of thepaired first novel optical switches is in input side and the right sideone of the paired first novel optical switches is in output side, thenthe switching operation of the left side one of the paired first noveloptical switches is carried out to select or switch any one of the twoinputs of the fifth novel optical switch having the two inputs and thetwo outputs, whilst the switching operation of the right side one of thepaired first novel optical switches is carried out to select or switchany one of the two outputs of the fifth novel optical switch having thetwo inputs and the two outputs, whereby the switching operations of thepaired first novel optical switches realize the fifth novel opticalswitch having the two inputs and the two outputs.

Each of the paired first novel optical switches is exactly the same asdescribed in the first embodiment, for which reason duplicatedescriptions to the first novel optical switches will be omitted.

As a modification to the above fifth novel optical switch, it is alsopossible that the fifth novel optical switch comprises a pair of theabove fourth novel optical switches described in the fourth embodiment.The two fourth novel optical switches are connected to each otherthrough a first optical transmission line 110 as a common line. If theleft side one of the paired fourth novel optical switches is in inputside and the right side one of the paired fourth novel optical switchesis in output side, then the switching operation of the left side one ofthe paired fourth novel optical switches is carried out to select orswitch any one of the two inputs of the fifth novel optical switchhaving the two inputs and the two outputs, whilst the switchingoperation of the right side one of the paired fourth novel opticalswitches is carried out to select or switch any one of the two outputsof the fifth novel optical switch having the two inputs and the twooutputs, whereby the switching operations of the paired fourth noveloptical switches realize the fifth novel optical switch having the twoinputs and the two outputs.

Each of the paired fourth novel optical switches is exactly the same asdescribed in the fourth embodiment, for which reason duplicatedescriptions to the fourth novel optical switches will be omitted.

In the above embodiment, the number of the wavelength multiplexing oneach optical transmission line is one. Notwithstanding, 8, 16, 32,64-wavelength multiplexing are available, wherein the batch-switchingoperation to the plural number wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal atnot only 1550 nanometers but also other wavelengths, for example, 1330nanometers.

It is also possible to set the wavelength of the excitation light at notonly 1480 nanometers or 980 nanometers but also the wavelengths providedthat such wavelength is capable of exciting the impurity doped fiber. Itis preferable to set the wavelength of the excitation light inconsideration of both the wavelength of the optical input signal and thekind of the impurity doped fiber.

The above excitation light switch may also be replaced by anacousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical outputsignal by controlling an optical power of the excitation light to be fedto the impurity doped fiber. It is possible to control the optical powerof the excitation light to be fed to the impurity doped fiber bycontrolling an injection current to the excitation light source or byuse of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rareearth doped fiber such as tellurium doped fiber. The length of the rareearth doped fiber and a doping concentration thereof may be set inaccordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rareearth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing todifferent excitation lights emitted separately from plural differentexcitation light sources in order to input the polarization-multiplexedexcitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical division at theoptical coupler in accordance with the various design choices.

The provisions of the smaller number of the excitation light source andthe single excitation light switch permit ON-OFF switching operations ofthe plural gate switches by a simple structure. The above switchexhibits such a gain property as a sharp rising, for which reason thereis substantially no influence due to a leaked light from the excitationlight switch. This makes the switch available to switches havingrelatively large crosstalk levels such as a polymer type switch orLiNbO.sub.3 switch, thereby realizing a low crosstalk and low insertionloss optical switch. In addition, the use of the impurity doped fiberserving as an optical power amplifier can obtain a gain as the opticalswitch.

Sixth Embodiment

A sixth embodiment according to the present invention will be describedin detail with reference to FIG. 6 which is a diagram illustrative of asixth novel optical switch having two inputs and two outputs. Astructural difference of the sixth novel optical switch from the fifthnovel optical switch is in providing a single or common excitation lightsource to a pair of modified first novel optical switches excludingindividual excitation light sources described in the first embodiment.

The sixth novel optical switch comprises a pair of the first noveloptical switches described in the first embodiment. The two first noveloptical switches are connected to each other through a first opticaltransmission line 110 as a common line. The two first novel opticalswitches are connected are also connected to the single and commonexcitation light source 31 to reduce the number of the requiredexcitation light source. If the left side one of the paired first noveloptical switches is in input side and the right side one of the pairedfirst novel optical switches is in output side, then the switchingoperation of the left side one of the paired first novel opticalswitches is carried out to select or switch any one of the two inputs ofthe sixth novel optical switch having the two inputs and the twooutputs, whilst the switching operation of the right side one of thepaired first novel optical switches is carried out to select or switchany one of the two outputs of the sixth novel optical switch having thetwo inputs and the two outputs, whereby the switching operations of thepaired first novel optical switches realize the sixth novel opticalswitch having the two inputs and the two outputs.

Each of the paired first novel optical switches is the same as describedin the first embodiment except for excluding the individual excitationlight sources, for which reason duplicate descriptions to the firstnovel optical switches will be omitted.

As a modification to the above sixth novel optical switch, it is alsopossible that the sixth novel optical switch comprises a pair of theabove fourth novel optical switches described in the fourth embodiment.The two fourth novel optical switches are connected to each otherthrough a first optical transmission line 110 as a common line. The twofirst novel optical switches are connected are also connected to thesingle and common excitation light source 31 to reduce the number of therequired excitation light source. If the left side one of the pairedfourth novel optical switches is in input side and the right side one ofthe paired fourth novel optical switches is in output side, then theswitching operation of the left side one of the paired fourth noveloptical switches is carried out to select or switch any one of the twoinputs of the sixth novel optical switch having the two inputs and thetwo outputs, whilst the switching operation of the right side one of thepaired fourth novel optical switches is carried out to select or switchany one of the two outputs of the sixth novel optical switch having thetwo inputs and the two outputs, whereby the switching operations of thepaired fourth novel optical switches realize the sixth novel opticalswitch having the two inputs and the two outputs.

Each of the paired fourth novel optical switches is exactly the same asdescribed in the fourth embodiment, for which reason duplicatedescriptions to the fourth novel optical switches will be omitted.

In the above embodiment, the number of the wavelength multiplexing oneach optical transmission line is one. Notwithstanding, 8, 16, 32,64-wavelength multiplexing are available, wherein the batch-switchingoperation to the plural number wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal atnot only 1550 nanometers but also other wavelengths, for example, 1330nanometers.

It is also possible to set the wavelength of the excitation light at notonly 1480 nanometers or 980 nanometers but also other wavelengthsprovided that such wavelength is capable of exciting the impurity dopedfiber. It is preferable to set the wavelength of the excitation light inconsideration of both the wavelength of the optical input signal and thekind of the impurity doped fiber.

The above excitation light switch may also be replaced by anacousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical outputsignal by controlling an optical power of the excitation light to be fedto the impurity doped fiber. It is possible to control the optical powerof the excitation light to be fed to the impurity doped fiber bycontrolling an injection current to the excitation light source or byuse of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rareearth doped fiber such as tellurium doped fiber. The length of the rareearth doped fiber and a doping concentration thereof may be set inaccordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rareearth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing todifferent excitation lights emitted separately from plural differentexcitation light sources in order to input the polarization-multiplexedexcitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical division at theoptical coupler in accordance with the various design choices.

The provisions of the smaller number of the excitation light source andthe single excitation light switch permit ON-OFF switching operations ofthe plural gate switches by a simple structure. The above switchexhibits such a gain property as a sharp rising, for which reason thereis substantially no influence due to a leaked light from the excitationlight switch. This makes the switch available to switches havingrelatively large crosstalk levels such as a polymer type switch orLiNbO3 switch, thereby realizing a low crosstalk and low insertion lossoptical switch. In addition, the use of the impurity doped fiber servingas an optical power amplifier can obtain a gain as the optical switch.

Seventh Embodiment

A seventh embodiment according to the present invention will bedescribed in detail with reference to FIG. 7 which is a diagramillustrative of a seventh novel optical switch having four separateoptical transmission lines for separately switching optical signaltransmission on the four separate optical transmission lines.

The seventh novel optical switch has first, second, third and fourthoptical transmission lines 1, 2, 3, and 4 on which separate opticalsignals are transmitted. The seventh novel optical switch also has anexcitation light source 31 for emitting an excitation light. The seventhnovel optical switch also has an excitation light switch 41 having asingle input connected to the excitation light source 31 and fouroutputs. The excitation light switch 41 is capable of separately ON-OFFswitching operations to transmissions of the excitation lights from thefour outputs.

The first optical transmission line 1 comprises a first input sideoptical transmission line 211 and a first output side opticaltransmission line 221, wherein the first input side optical transmissionline 211 is connected through a first erbium doped fiber 11EDF to thefirst input side optical transmission line 211. A first optical coupler21 is provided on the first input side optical transmission line 211.The first optical coupler 21 is connected to a first output of theexcitation light switch 41. A first optical signal is transmitted on thefirst input side optical transmission line 211 through the first erbiumdoped fiber 11EDF to the first input side optical transmission line 211.If the excitation light switch 41 is operated to switch ON to allowtransmission of the excitation light emitted from the excitation lightsource 31 through the first optical coupler 21 to the first erbium dopedfiber 11EDF, then the first erbium doped fiber 11EDF is excited to allowthat the transmission of the first optical signal having beentransmitted on the first input side optical transmission line 211 istransmitted through the first erbium doped fiber 11EDF to the firstoutput side optical transmission line 221.

The second optical transmission line 1 comprises a second input sideoptical transmission line 212 and a second output side opticaltransmission line 222, wherein the second input side opticaltransmission line 212 is connected through a second erbium doped fiber12EDF to the second input side optical transmission line 212. A secondoptical coupler 22 is provided on the second input side opticaltransmission line 212. The second optical coupler 22 is connected to asecond output of the excitation light switch 41. A second optical signalis transmitted on the second input side optical transmission line 212through the second erbium doped fiber 12EDF to the second input sideoptical transmission line 212. If the excitation light switch 41 isoperated to switch ON to allow transmission of the excitation lightemitted from the excitation light source 31 through the second opticalcoupler 22 to the second erbium doped fiber 12EDF, then the seconderbium doped fiber 12EDF is excited to allow that the transmission ofthe second optical signal having been transmitted on the second inputside optical transmission line 212 is transmitted through the seconderbium doped fiber 12EDF to the second output side optical transmissionline 222.

The third optical transmission line 1 comprises a third input sideoptical transmission line 213 and a third output side opticaltransmission line 223, wherein the third input side optical transmissionline 213 is connected through a third erbium doped fiber 13EDF to thethird input side optical transmission line 213. A third optical coupler23 is provided on the third input side optical transmission line 213.The third optical coupler 23 is connected to a third output of theexcitation light switch 41. A third optical signal is transmitted on thethird input side optical transmission line 213 through the third erbiumdoped fiber 13EDF to the third input side optical transmission line 213.If the excitation light switch 41 is operated to switch ON to allowtransmission of the excitation light emitted from the excitation lightsource 31 through the third optical coupler 23 to the third erbium dopedfiber 13EDF, then the third erbium doped fiber 13EDF is excited to allowthat the transmission of the third optical signal having beentransmitted on the third input side optical transmission line 213 istransmitted through the third erbium doped fiber 13EDF to the thirdoutput side optical transmission line 223.

The fourth optical transmission line 1 comprises a fourth input sideoptical transmission line 214 and a fourth output side opticaltransmission line 224, wherein the fourth input side opticaltransmission line 214 is connected through a fourth erbium doped fiber14EDF to the fourth input side optical transmission line 214. A fourthoptical coupler 24 is provided on the fourth input side opticaltransmission line 214. The fourth optical coupler 24 is connected to afourth output of the excitation light switch 41. A fourth optical signalis transmitted on the fourth input side optical transmission line 214through the fourth erbium doped fiber 14EDF to the fourth input sideoptical transmission line 214. If the excitation light switch 41 isoperated to switch ON to allow transmission of the excitation lightemitted from the excitation light source 31 through the fourth opticalcoupler 24 to the fourth erbium doped fiber 14EDF, then the fourtherbium doped fiber 14EDF is excited to allow that the transmission ofthe fourth optical signal having been transmitted on the fourth inputside optical transmission line 214 is transmitted through the fourtherbium doped fiber 14EDF to the fourth output side optical transmissionline 224.

The excitation light switch 41 is capable of separate ON-OFF switchingoperations to the four outputs from which the excitation lights areoutputted. If the excitation light switch 41 is operated to switch ON tothe four outputs, then the excitation lights are red through the first,second, third and fourth couplers 21, 22, 23 and 24 to the first,second, third and fourth erbium doped fibers 11EDF, 12EDF, 13EDF and14EDF, whereby the first, second, third and fourth optical signals aretransmitted through the first, second, third and fourth erbium dopedfibers 11EDF, 12EDF, 13EDF and 14EDF to the first, second, third andfourth output side optical transmission lines 221, 222, 223 and 224. Ifthe excitation light switch 41 is operated to switch ON to the first,second and third outputs, then the excitation lights are fed through thefirst, second and third couplers 21, 22 and 23 to the first, second andthird erbium doped fibers 11EDF, 12EDF and 13EDF, whereby the first,second and third optical signals are transmitted through the first,second and third erbium doped fibers 11EDF, 12EDF and 13EDF to thefirst, second and third output side optical transmission lines 221, 222and 223, whilst the fourth optical signal is absorbed by the fourtherbium doped fiber 14EDF. If the excitation light switch 41 is operatedto switch ON to the first and second outputs, then the excitation lightsare fed through the first and second couplers 21 and 22 to the first andsecond erbium doped fibers 11EDF and 12EDF, whereby the first and secondoptical signals are transmitted through the first and second erbiumdoped fibers 11EDF and 12EDF to the first and second output side opticaltransmission lines 221 and 222, whilst the third and fourth opticalsignals are absorbed by the third and fourth erbium doped fibers 13EDFand 14EDF. If the excitation light switch 41 is operated to switch ON tothe first output, then the excitation lights are fed through the firstcoupler 21 to the first erbium doped fiber 11EDF, whereby the firstoptical signal is transmitted through the first erbium doped fiber 11EDFto the first output side optical transmission line 221, whilst thesecond, third and fourth optical signals are absorbed by the second,third and fourth erbium doped fibers 12EDF, 13EDF and 14EDF.

In the above embodiment, the number of the wavelength multiplexing oneach optical transmission line is one. Notwithstanding, 8, 16, 32,64-wavelength multiplexing are available, wherein the batch-switchingoperation to the plural number wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal atnot only 1550 nanometers but also other wavelengths, for example, 1330nanometers.

It is also possible to set the wavelength of the excitation light at notonly 1480 nanometers or 980 nanometers but also other wavelengthsprovided that such wavelengths is capable of exciting the impurity dopedfiber. It is preferable to set the wavelength of the excitation light inconsideration of both the wavelength of the optical input signal and thekind of the impurity doped fiber.

The above excitation light switch may also be replaced by anacousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical outputsignal by controlling an optical power of the excitation light to be fedto the impurity doped fiber. It is possible to control the optical powerof the excitation light to be fed to the impurity doped fiber bycontrolling an injection current to the excitation light source or byuse of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rareearth doped fiber such as tellurium doped fiber. The length of the rareearth doped fiber and a doping concentration thereof may be set inaccordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rareearth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing todifferent excitation lights emitted separately from plural differentexcitation light sources in order to input the polarization-multiplexedexcitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical division at theoptical coupler in accordance with the various design choices.

The provisions of the smaller number of the excitation light source andthe single excitation light switch permit ON-OFF switching operations ofthe plural gate switches by a simple structure. The above switchexhibits such a gain property as a sharp rising, for which reason thereis substantially no influence due to a leaked light from the excitationlight switch. This makes the switch available to switches havingrelatively large crosstalk levels such as a polymer type switch orLiNbO3 switch, thereby realizing a low crosstalk and low insertion lossoptical switch. In addition, the use of the impurity doped fiber servingas an optical power amplifier can obtain a gain as the optical switch.

Eighth Embodiment

An eighth embodiment according to the present invention will bedescribed in detail with reference to FIG. 8 which is a diagramillustrative of an eighth novel optical switch having four separateoptical transmission lines for separately switching optical signaltransmissions on the four separate optical transmission lines. Astructural difference of the eighth novel optical switch from theseventh novel optical switch is in providing double excitation lightsources and an optical cross connector serving as a switch.

The eighth novel optical switch has first, second, third and fourthoptical transmission lines 1, 2, 3, and 4 on which separate opticalsignals are transmitted. The seventh novel optical switch also has firstand second excitation light sources 32 and 33 for emitting excitationlights. The eighth novel optical switch also has an optical crossconnector 81 serving as a switch having two inputs connected to thefirst and second excitation light sources 32 and 33 and four outputs.The optical cross connector 81 is capable of separate switchingoperations of the four outputs for each of the excitation lights emittedfrom the first and second excitation light sources 32 and 33. The dualexcitation lights sources 32 and 33 increases the excitation power to befed to the individual erbium doped fibers on the first to fourth opticaltransmission lines 1, 2, 3, and 4.

The first optical transmission line 1 comprises a first input sideoptical transmission line 211 and a first output side opticaltransmission line 221, wherein the first input side optical transmissionline 211 is connected through a first erbium doped fiber 11EDF to thefirst input side optical transmission line 211. A first optical coupler21 is provided on the first input side optical transmission line 211.The first optical coupler 21 is connected to a first output of theoptical cross connector 81. A first optical signal is transmitted on thefirst input side optical transmission line 211 through the first erbiumdoped fiber 11EDF to the first input side optical transmission line 211.If the optical cross connector 81 is operated to switch ON to allowtransmission of the excitation light emitted from the excitation lightsource 31 through the first optical coupler 21 to the first erbium dopedfiber 11EDF, then the first erbium doped fiber 11EDF is excited to allowthat the transmission of the first optical signal having beentransmitted on the first input side optical transmission line 211 istransmitted through the first erbium doped fiber 11EDF to the firstoutput side optical transmission line 221.

The second optical transmission line 1 comprises a second input sideoptical transmission line 212 and a second output side opticaltransmission line 222, wherein the second input side opticaltransmission line 212 is connected through a second erbium doped fiber12EDF to the second input side optical transmission line 212. A secondoptical coupler 22 is provided on the second input side opticaltransmission line 212. The second optical coupler 22 is connected to asecond output of the optical cross connector 81. A second optical signalis transmitted on the second input side optical transmission line 212through the second erbium doped fiber 12EDF to the second input sideoptical transmission line 212. If the optical cross connector 81 isoperated to switch ON to allow transmission of the excitation lightemitted from the excitation light source 31 through the second opticalcoupler 22 to the second erbium doped fiber 12EDF, then the seconderbium doped fiber 12EDF is excited to allow that the transmission ofthe second optical signal having been transmitted on the second inputside optical transmission line 212 is transmitted through the seconderbium doped fiber 12EDF to the second output side optical transmissionline 222.

The third optical transmission line 1 comprises a third input sideoptical transmission line 213 and a third output optical transmissionline 223, wherein the third input side optical transmission line 213 isconnected through a third erbium doped fiber 13EDF to the third inputside optical transmission line 213. A third optical coupler 23 isprovided on the third input side optical transmission line 213. Thethird optical coupler 23 is connected to a third output of the opticalcross connector 81. A third optical signal is transmitted on the thirdinput side optical transmission line 213 through the third erbium dopedfiber 13EDF to the third input side optical transmission line 213. Ifthe optical cross connector 81 is operated to switch ON to allowtransmission of the excitation light emitted from the excitation lightsource 31 through the third optical coupler 23 to the third erbium dopedfiber 13EDF, then the third erbium doped fiber, 13EDF is excited toallow that the transmission of the third optical signal having beentransmitted on the third input side optical transmission line 213 istransmitted through the third erbium doped fiber 13EDF to the thirdoutput side optical transmission line 223.

The fourth optical transmission line 1 comprises a fourth input sideoptical transmission line 214 and a fourth output side opticaltransmission line 224, wherein the fourth input side opticaltransmission line 214 is connected through a fourth erbium doped fiber14EDF to the fourth input side optical transmission line 214. A fourthoptical coupler 24 is provided on the fourth input side opticaltransmission line 214. The fourth optical coupler 24 is connected to afourth output of the optical cross connector 81. A fourth optical signalis transmitted on the fourth input side optical transmission line 214through the fourth erbium doped fiber 14EDF to the fourth input sideoptical transmission line 214. If the optical cross connector 81 isoperated to switch ON to allow transmission of the excitation lightemitted from the excitation light source 31 through the fourth opticalcoupler 24 to the fourth erbium doped fiber 14EDF, then the fourtherbium doped fiber 14EDF is excited to allow that the transmission ofthe fourth optical signal having been transmitted on the fourth inputside optical transmission line 214 is transmitted through the fourtherbium doped fiber 14EDF to the fourth output side optical transmissionline 224.

The optical cross connector 81 is capable of separate ON-OFF switchingoperations to the four outputs from which the excitation lights areoutputted. If the optical cross connector 81 is operated to switch ON tothe four outputs, then the excitation lights are fed through the first,second, third and fourth couplers 21, 22, 23 and 24 to the first,second, third and fourth erbium doped fibers 11EDF, 12EDF, 13EDF and14EDF, whereby the first, second, third and fourth optical signals aretransmitted through the first, second, third and fourth erbium dopedfibers 11EDF, 12EDF, 13EDF and 14EDF to the first, second, third andfourth output side optical transmission lines 221, 222, 223 and 224. Ifthe optical cross connector 81 is operated to switch ON to the first,second and third outputs, then the excitation lights are fed through thefirst, second and third couplers 21, 22 and 23 to the first, second andthird erbium doped fibers 11EDF, 12EDF and 13EDF, whereby the first,second and third optical signals are transmitted through the first,second and third erbium doped fibers 11EDF, 12EDF and 13EDF to thefirst, second and third output side optical transmission lines 221, 222and 223, whilst the fourth optical signal is absorbed by the fourtherbium doped fiber 14EDF. If the optical cross connector 81 is operatedto switch ON to the first and second outputs, then the excitation lightsare fed through the first and second couplers 21 and 22 to the first andsecond erbium doped fibers 11EDF and 12EDF, whereby the first and secondoptical signals are transmitted through the first and second erbiumdoped fibers 11EDF and 12EDF to the first and second output side opticaltransmission lines 221 and 222, whilst the third and fourth opticalsignals are absorbed by the third and fourth erbium doped fibers 13EDFand 14EDF. If the optical cross connector 81 is operated to switch ON tothe first output, then the excitation lights are fed through the firstcoupler 21 to the first erbium doped fiber 11EDF, whereby the firstoptical signal is transmitted through the first erbium doped fiber 11EDFto the first output side optical transmission line 221, whilst thesecond, third and fourth optical signals are absorbed by the second,third and fourth erbium doped fibers 12EDF, 13EDF and 14EDF.

The duel excitation light sources 32 and 33 increases the excitationpower to be fed to the individual erbium doped fibers on the first tofourth optical transmission lines 1, 2, 3 and 4. This means it possibleto further increase the number of the separate optical transmissionlines to increase the size of the optical switch.

In the above embodiment, the number of the wavelength multiplexing oneach optical transmission line is one. Notwithstanding, 8, 16, 32,64-wavelength multiplexing are available, wherein the batch-switchingoperation to the plural member wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal atnot only 1550 nanometers but also other wavelengths, for example, 1330nanometers.

It is also possible to set the wavelength of the excitation light at notonly 1480 nanometers or 980 nanometers but also other wavelengthsprovided that such wavelengths is capable of exciting the impurity dopedfiber. It is preferable to set the wavelength of the excitation light inconsideration of both the wavelength of the optical input signal and thekind of the impurity doped fiber.

The above excitation light switch may also be replaced by anacousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical outputsignal by controlling an optical power of the excitation light to be fedto the impurity doped fiber. It is possible to control the optical powerof the excitation light to be fed to the impurity doped fiber bycontrolling an injection current to the excitation light source or byuse of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rareearth doped fiber such as tellurium doped fiber. The length of the rareearth doped fiber and a doping concentration thereof may be set inaccordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rareearth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing todifferent excitation lights emitted separately from plural differentexcitation light sources in order to input the polarization-multiplexedexcitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical division at theoptical coupler in accordance with the various design choices.

The provisions of the smaller number of the excitation light source andthe single excitation light switch permit ON-OFF switching operations ofthe plural gate switches by a simple structure. The above switchexhibits such a gain property as a sharp rising, for which reason thereis substantially no influence due to a leaked light from the excitationlight switch. This makes the switch available to switches havingrelatively large crosstalk levels such as polymer type switch or LiNbO3switch, thereby realizing a low crosstalk and low insertion loss opticalswitch. In addition, the use of the impurity doped fiber serving as anoptical power amplifier can obtain a gain as the optical switch.

Ninth Embodiment

A ninth embodiment according to the present invention will be describedin detail with reference to FIG. 9 which is a diagram illustrative of aninth novel optical switch provided in a novel first optical add-dropmultiplexer performing optical addition, drop and transmission of saidoptical signals.

The ninth novel optical switch comprises a first optical transmissionline 110 for transmitting an optical signal having a wavelength of 1550nanometers, an optical reflectivity variable mirror 50 connected to thefirst optical transmission line 110 for reflecting the optical signal ata controlled reflectivity, a second optical transmission line 120connected to the optical reflectivity variable mirror 50, and an opticaltransmitter 81 connected through the second optical transmission line120 to the optical reflectivity variable mirror 50 for transmitting anoptical signal having a wavelength of 1550 nanometers.

In order to form the novel first optical add-drop multiplexer, a firstoptical coupler 11 is provided on the first optical transmission line110. Further, a third optical transmission line 121 is connected to thefirst optical coupler 11. Furthermore, an optical receiver 71 is alsoconnected with the third optical transmission line 121 so that theoptical receiver 71 is also connected through the third opticaltransmission line 121 to the first optical coupler 11. The optical inputsignal is divided by the first optical coupler 11 so that one of thedivided optical input signals is transmitted through the third opticaltransmission line 121 to the optical receiver 71, whilst the remainingone of the divided optical input signals is transmitted to the opticalreflectivity variable mirror 50 whereby the remaining one of the dividedoptical input signals is reflected by the optical reflectivity variablemirror 50 at a controlled reflectivity. The optical reflectivityvariable mirror 50 is capable of varying a reflectivity in the range offrom 0% to 100%. If the reflectivity of the optical reflectivityvariable mirror 50 is set 0%, then the optical reflectivity variablemirror 50 is a transmission state which allows an optical signaltransmission. In this case, the optical signal transmitted from theoptical transmitter 81 is transmitted through the optical reflectivityvariable mirror 50 to the first optical transmission line.

A signal transmission operation of the novel first optical add-dropmultiplexer will subsequently be described. An optical input signalhaving a wavelength of 1550 nanometers is transmitted on the firstoptical transmission line 110 and then reflected by the opticalreflectivity variable mirror 50 before the reflected optical signal isthen transmitted on the first optical transmission line 110.

A signal drop operation of the novel first optical add-drop multiplexerwill subsequently be described. An optical input signal having awavelength of 1550 nanometers is transmitted on the first opticaltransmission line 110 and then divided into two parts by the opticalcoupler 11. One of the divided optical input signals is then transmittedthrough the third optical transmission line 121 to the optical receiver71. It is possible to set a low ratio of a first optical division forthe optical receiver 71 to a second optical division for the opticalreflectivity variable mirror 50, in order to suppress an optical loss bythe optical division by the optical coupler 11.

A signal add operation of the novel first optical add-drop multiplexerwill subsequently be described. The optical reflectivity variable mirror50 is capable of varying a reflectivity in the range of from 0% to 100%.If the reflectivity of the optical reflectivity variable mirror 50 isset 0%, then the optical reflectivity variable mirror 50 is in atransmission state which allows an optical signal transmission. In thiscase, the optical signal transmitted from the optical transmitter 81 istransmitted through the optical reflectivity variable mirror 50 to thefirst optical transmission line.

The above novel first optical add-drop multiplexer does require nooptical coupler for signal adding, thereby realizing a low optical loss.

As a modification to this embodiment, it is possible to provide any oneof the above first to fourth optical switches of FIGS. 1 to 4 in thefirst to fourth embodiments, in place of the above optical coupler 11and the optical reflectivity variable mirror 50.

It is possible that the input and output ports are commonly used or thatthe input and output ports are separated from each other by use of anoptical coupler and an optical isolator or by use of a circulator.

It is also possible that the optical reflectivity variable mirror 50 maybe replaced by an optical switch for switching an transmission and areflection, or by an optical reflectivity switching mirror for switching0% reflectivity and 100% reflectivity, provided that if the reflectivityis 0%, then the switch is capable of transmission of the optical signal.

It is also possible to change the number of the gate arrays from eight.

The optical multiplexer, the optical demultiplexer or the opticalmultiplexer/demultiplexer may comprise an array waveguide grating, awavelength router having substantially the same grating structure as thearray waveguide grating, or a wavelength MUX coupler havingsubstantially the same grating structure as the array waveguide grating.

Since insertion loss is different among the optical multiplexer, theoptical demultiplexer and the optical multiplexer/demultiplexer, it ispossible to use optical attenuators in individual waveguides for controlof the optical power levels.

It is also possible to control a gain of the erbium doped fiberamplifier gate or control reflectivity of the reflective mirror forcontrol of the optical power levels for every wavelengths separately.

It is furthermore possible to replace the erbium doped fiber by rareearth doped fiber such as tellurium doped fiber, or an aluminum dopedfiber. The length of the rare earth doped fiber and a dopingconcentration thereof may be set in accordance with the requiredspecifications of the optical switch.

The excitation light may have a wavelength of 980 nanometers in order toshorten the wavelength for a remarkable reduction in noise factor of theoptical output signal. In this case, the optical switch is also freefrom any substantive insertion loss and a low or reduced crosstalk.

Tenth Embodiment

A tenth embodiment according to the present invention will be describedin detail with reference to FIG. 10 which is a diagram illustrative of atenth novel optical switch as an optical gate switch.

The tenth novel optical switch is an optical gate switch. The tenthnovel optical switch comprises an optical input signal transmission line110 for transmitting an optical input signal, an optical output signaltransmission line 120 for transmitting an optical output signal, anoptical transmission line 130 connected through an optical coupler 11 tosaid input and output signal transmission lines 110 and 120, an opticalisolator 91 provided on said optical input signal transmission line 110for permitting a unidirectional transmission of said optical inputsignal toward said optical transmission line 130, an erbium doped fiber41 provided on said optical transmission line 130, a wavelength bandselective optical reflecting mirror 25 provided on said opticaltransmission line 130, and an excitation light source 31 connected tosaid wavelength band selective optical reflecting mirror 25. The opticalinput signal has a wavelength of 1550 nanometers. The excitation lightsource 31 is capable of emitting an excitation light having a wavelengthof 1480 nanometers. The wavelength band selective optical reflectingmirror 25 is capable of selecting a reflecting wavelength band of anoptical signal to be reflected by the wavelength band selective opticalreflecting mirror 25.

In this case, the wavelength band selective optical reflecting mirror 25so sets the reflecting wavelength band that the optical input signalwith the wavelength of 1550 nanometers is total-reflected by thewavelength band selective optical reflecting mirror 25, whilst theexcitation light emitted from the excitation light source 31 istransmitted through the wavelength band selective optical reflectingmirror 25 to the erbium doped fiber 41, whereby the erbium doped fiber41 is excited by the excitation light. The excited erbium doped fiber 41is capable of amplifying the optical input signal. The amplified inputsignal is then total-reflected by the wavelength band selective opticalreflecting mirror 25. The reflected input signal is then transmittedagain through the erbium doped fiber 41, whereby the reflected signal isfurther amplified. The further amplified optical signal is divided bythe optical coupler 11 into two parts, one of which is transmitted tothe optical isolator 91. However, the transmission of the dividedoptical signal is prevented by the optical isolator 91. On the otherhand, the other divided part of the optical signal is transmittedthrough the output signal transmission line 120. In the above state, theabove optical gate switch is in ON state.

If no excitation light is emitted from the excitation light source 31,the erbium doped fiber 41 receives no excitation light and is unexcited,whereby the input optical signal is absorbed by the erbium doped fiber41. No optical signal is outputted from the output signal transmissionline 120. In the above state, the optical gate switch is in OFF state.

The above novel optical gate switch is capable of reducing an insertionloss and also reducing the number of the required optical couplers.

It is also possible to integrate the wavelength band selective opticalreflecting mirror 25 and the excitation light source 31. FIG. 11 is aschematic view illustrative of an integration of the above novel opticalgate switch. An erbium doped fiber amplifier gate array 300 has anintegration of eight sets of an excitation light source 31, a wavelengthband selective optical reflecting mirror 25, an erbium doped fiber 41,and an optical transmission line 100.

As a modification to this embodiment, it is possible to provide any oneof the above first to fourth optical switches of FIGS. 1 to 4 in thefirst to fourth embodiments, in place of the above optical coupler 11and the optical reflectivity variable mirror 50.

It is possible that the input and output ports are commonly used or thatthe input and output ports are separated from each other by use of anoptical coupler and an optical isolator or by use of a circulator.

It is possible to change the positions of the optical couplers providedthat the functions of the optical add-drop multiplexer can be ensured.

The optical multiplexer, the optical demultiplexer or the opticalmultiplexer/demultiplexer may comprise an array waveguide grating, awavelength router having substantially the same grating structure as thearray waveguide grating, or a wavelength MUX coupler havingsubstantially the same grating structure as the array waveguide grating.

Since insertion loss is different among the optical multiplexer, theoptical demultiplexer and the optical multiplexer/demultiplexer, it ispossible to use optical attenuators individual waveguides for control ofthe optical power levels.

It is also possible to control a gain of the erbium doped fiberamplifier gate or control reflectivity of the reflective mirror forcontrol of the optical power levels for every wavelengths separately.

It is furthermore possible to replace the erbium doped fiber by rareearth doped fiber such as tellurium doped fiber, or an aluminum dopedfiber. The length of the rare earth doped fiber and a dopingconcentration thereof may be set in accordance with the requiredspecifications of the optical switch.

The excitation light may have a wavelength of 980 nanometers in order toshorten the wavelength of a remarkable reduction in noise factor of theoptical output signal. In this case, the optical switch is also freefrom any substantive insertion loss and a low or reduced crosstalk.

Eleventh Embodiment

An eleventh embodiment according to the present invention will bedescribed in detail with reference to FIG. 12 which is a diagramillustrative of a novel optical add-drop multiplexer using an opticalgate switch of FIG. 10 for performing optical addition, drop andtransmission of said optical signals.

The novel optical add-drop multiplexer using an optical gate switchcomprises an optical input signal transmission line 110 for transmittingan optical input signal, an optical output signal transmission line 120for transmitting an optical output signal, an optical transmission line130 connected through an optical coupler 11 to said input and outputsignal transmission lines 110 and 120, and optical isolator 91 providedon said optical input signal transmission line 110 for permitting aunidirectional transmission of said optical input signal toward saidoptical transmission line 130, an erbium doped fiber 41 provided on saidoptical transmission line 130, a wavelength band selective opticalreflecting mirror 25 provided on said optical transmission line 130, anexcitation light source 31 connected to said wavelength band selectiveoptical reflecting mirror 25, an optical receiver 71 connected through asecond optical coupler 12 to the optical transmission line 130 andpositioned between the first optical coupler 11 and the erbium dopedfiber 41 and an optical transmitter 81 connected through a third opticalcoupler 13 to the output signal optical transmission line 130. Theoptical input signal has a wavelength of 1550 nanometers. The excitationlight source 31 is capable of emitting an excitation light have awavelength of 1480 nanometers. The wavelength band selective opticalreflecting mirror 25 is capable of selecting a reflecting wavelengthband of an optical signal to be reflected by the wavelength bandselective optical reflecting mirror 25.

In this case, the wavelength band selective optical reflecting mirror 25so sets the reflecting wavelength band that the optical input signalwith the wavelength of 1550 nanometers is total-reflected by thewavelength band selective optical reflecting mirror 25, whilst theexcitation light emitted from the excitation light source 31 istransmitted through the wavelength band selective optical reflectingmirror 25 to the erbium doped fiber 41, whereby the erbium doped fiber41 is excited by the excitation light. The excited erbium doped fiber 41is capable of amplifying the optical input signal. The amplified inputsignal is then total-reflected by the wavelength band selective opticalreflecting mirror 25. The reflected input signal is then transmittedagain through the erbium doped fiber 41, whereby the reflected signal isfurther amplified. The further amplified optical signal is divided bythe optical coupler 11 into two parts, one of which is transmitted tothe optical isolator 91. However, the transmission of the dividedoptical signal is prevented by the optical isolator 91. On the otherhand, the other divided part of the optical signal is transmittedthrough the output signal transmission line 120. In the above state, theabove optical gate switch is in ON state.

A signal transmission operation of the above novel optical add-dropmultiplexer will subsequently be described. The excitation light isemitted from the excitation light source 31 and then supplied to theerbium doped fiber 41, whereby the erbium doped fiber 41 is excited. Theoptical input signal is transmitted through the erbium doped fiber 41and amplified by the excited erbium doped fiber 41. The amplifiedoptical input signal is total-reflected by the wavelength band selectiveoptical reflecting mirror 25. The reflected optical signal is thentransmitted again through the erbium doped fiber 41, whereby thereflected signal is further amplified. The further amplified opticalsignal is divided by the first optical coupler 11 into two parts, one ofwhich is transmitted to the optical isolator 91. However, thetransmission of the divided optical signal is prevented by the opticalisolator 91. On the other hand, the other divided part of the opticalsignal is transmitted through the output signal transmission line 120.

A signal drop operation of the above novel optical add-drop multiplexerwill subsequently be described. The optical input signal is divided bythe second optical coupler into two parts, one of which is transmittedto the optical receiver 71.

It is possible to set the ratio of first optical division for theoptical receiver 71 to second optical division for the erbium dopedfiber 41 is small in order to reduce the optical loss due to the secondoptical coupler 12.

A signal add operation of the above novel optical add-drop multiplexerwill subsequently be described. In this case, no excitation light isemitted from the excitation light source 31, for which reason the erbiumdoped fiber 41 receives 110 excitation light and is unexcited, wherebythe input output signal is absorbed by the erbium doped fiber 41. Nooptical signal is outputted from the output signal transmission line120. On the other hand, the optical transmitter 81 emits a secondoptical signal with a wavelength of 1550 nanometers which is thentransmitted on the output signal optical transmission line 120, wherebythe second optical signal is outputted from the output signal opticaltransmission line 120.

The above novel optical add-drop multiplexer is capable of reducing aninsertion loss and also reducing the number of required opticalcouplers.

It is also possible to integrate the wavelength band selective opticalreflecting mirror 25 and the excitation light source 31. FIG. 13 is aschematic view illustrative of an integration of the above novel opticaladd-drop multiplexer. An erbium doped fiber amplifier gate module 202 isprovided formed on a package 210. The erbium doped fiber amplifier gatemodule 202 has an erbium doped fiber amplifier gate array 300. Theerbium doped fiber amplifier gate array 300 has an integration of eightsets of an excitation light source 31, a wavelength band selectiveoptical reflecting mirror 25, an erbium doped fiber 41, and an opticaltransmission line 100. The erbium doped fiber amplifier gate module 202has an input port 110 and an output port 120 which are provided in thesame side of the package 201. This allows a high density integration ofthe package.

As a modification to this embodiment, it is possible to provide any oneof the above first to fourth optical switches of FIGS. 1 to 4 in thefirst to fourth embodiments, in place of the above optical gate switch.

It is possible that the input and output ports are commonly used or thatthe input and output ports are separated from each other by use of anoptical coupler and an optical isolator or by use of a circulator.

It is also possible that the erbium doped fibers are packaged in thesame array, or that the reflective mirrors are incorporated into theerbium doped fiber amplifier gate module, or that all of the aboveelements are packaged onto a PLC board.

The optical multiplexer, the optical demultiplexer or the opticalmultiplexer/demultiplexer may comprise an array waveguide grating, awavelength router having substantially the same grating structure as thearray waveguide grating, or a wavelength MUX coupler havingsubstantially the same grating structure as the array waveguide grating.

Since the insertion loss is different among the optical multiplexer, theoptical demultiplexer and the optical multiplexer/demultiplexer, it ispossible to use optical attenuators in individual waveguides for controlof the optical power levels.

It is also possible to control a gain of the erbium doped fiberamplifier gate or control reflectivity of the reflective mirror forcontrol of the optical power levels for every wavelengths separately.

It is furthermore possible to replace the erbium doped fiber by rareearth doped fiber such as tellurium doped fiber, or an aluminum dopedfiber. The length of the rare earth doped fiber and a dopingconcentration thereof may be set in accordance with the requiredspecifications of the optical switch.

The excitation light may have a wavelength of 980 nanometers in order toshorten the wavelength for a remarkable reduction in noise factor of theoptical output signal. In this case, the optical switch is also freefrom any substantive insertion loss and a low or reduced crosstalk.

Twelfth Embodiment

A twelfth embodiment according to the present invention will bedescribed in detail with reference to FIG. 14 which is a diagramillustrative of a novel wavelength-multiplexed optical add-dropmultiplexer using four sets of the above novel optical add-dropmultiplexer of FIG. 9.

The novel wavelength-multiplexed optical add-drop multiplexer comprisesa single optical circulator 60 connected with optical transmission lines110, 120 and 121, an optical multiplexer/demultiplexer 410 connectedthrough said optical transmission line 120 to said optical circulator 60and first to fourth optical add-drop multiplexer 1, 2, 3 and 4. Thefirst optical add-drop multiplexer is operable for a signal having awavelength of 1548 nanometers. The second optical add-drop multiplexeris operable for a signal having a wavelength of 1550 nanometers. Thethird optical add-drop multiplexer is operable for a signal having awavelength of 1552 nanometers. The fourth optical add-drop multiplexeris operable for a signal having a wavelength of 1554 nanometers.

The optical input signal having four wavelength compositions of 1548nanometers, 1550 nanometers, 1552 nanometers, and 1554 nanometers istransmitted from the optical transmission line 110 through the opticalcirculator 60 to the optical multiplexer/demultiplexer 410, so that theoptical input signal is wavelength-demultiplexer by the opticalmultiplexer/demultiplexer 410 whereby the optical input signal isdivided into a first signal having a wavelength of 1548 nanometers, asecond signal having a wavelength of 1550 nanometers, a third signalhaving a wavelength of 1552 nanometers, and a fourth signal havingwavelength of 1554 nanometers. The first, second, third and fourthoptical signals are inputted into the first, second, third and fourthoptical add-drop multiplexers 1, 2, 3 and 4 respectively.

The first optical add-drop multiplexer 1 comprises a first main opticaltransmission line 131 for transmitting the first optical signal, a firstoptical reflectivity variable mirror 51 provided on the first mainoptical transmission line 131 for reflecting the first optical signal ata controlled reflectivity, and an optical transmitter 81 with an end ofthe first main optical transmission line 131, a first subordinateoptical transmission line 135 connected through a first optical coupler11 to the first main optical transmission line, and a first opticalreceiver 71 connected with said first subordinate optical transmissionline 135. The first optical input signal is divided by the first opticalcoupler 11 so that one of the divider first optical input signal istransmitted the first subordinate optical transmission line 135 to theoptical receiver 71, whilst the remaining one of the divided firstoptical input signal is transmitted to the optical reflectivity variablemirror 51 whereby the remaining one of the divided first optical inputsignal is reflected by the first optical reflectivity variable mirror 51at a controlled reflectivity. The first optical reflectivity variablemirror 51 is capable of varying a reflectivity in the range of from 0%to 100%. If the reflectivity of the first optical reflectivity variablemirror 51 is set 0%, then the first optical reflectivity variable mirror51 is in a transmission state which allows an optical signaltransmission. In this case, the first optical signal transmitted fromthe first optical transmitter 81 is transmitted through the firstoptical reflectivity variable mirror 51 to the first main opticaltransmission line 131.

A signal transmission operation of the first optical add-dropmultiplexer will subsequently be described. The first optical inputsignal is transmitted on the first main optical transmission line 131and then reflected by the first optical reflectivity variable mirror 51before the reflected optical signal is then transmitted on the firstmain optical transmission line 131.

A signal drop operation of the first optical add-drop multiplexer willsubsequently be described. The first optical input signal is transmittedon the first main optical transmission line 131 and then divided intotwo parts by the first optical coupler 11. One of the divided firstoptical input signals is then transmitted through the first subordinateoptical transmission line 135 to the optical receiver 71.

A signal add operation of the first optical add-drop multiplexer willsubsequently be described. If the reflectivity of the first opticalreflectivity variable mirror 51 is set 0%, then the first opticalreflectivity variable mirror 51 is in a transmission state which allowsan optical signal transmission. In this case, a first substitute opticalsignal transmitted from the optical transmitter 81 is transmittedthrough the first optical reflectivity variable mirror 51 to the firstmain optical transmission line 131.

The second optical add-drop multiplexer 2 comprises a second mainoptical transmission line 132 for transmitting the second opticalsignal, a second optical reflectivity variable mirror 52 provided on thesecond main optical transmission line 132 for reflecting the secondoptical signal at a controlled reflectivity, and an optical transmitter82 with an end of the second main optical transmission line 132, asecond subordinate optical transmission line 136 connected through asecond optical coupler 12 to the second main optical transmission line,and a second optical receiver 72 connected with said second subordinateoptical transmission line 136. The second optical input signal isdivided by the second optical coupler 12 so that one of the dividedsecond optical input signal is transmitted through the secondsubordinate optical transmission line 136 to the optical receiver 72,whilst the remaining one of the divided second optical input signal istransmitted to the optical reflectivity variable mirror 52 whereby theremaining one of the divided second optical input signal is reflected bythe second optical reflectivity variable mirror 52 at a controlledreflectivity. The second optical reflectivity variable mirror 52 iscapable of varying a reflectivity in the range of from 0% to 100%. Ifthe reflectivity of the second optical reflectivity variable mirror 52is set 0%, then the second optical reflectivity variable mirror 52 is ina transmission state which allows an optical signal transmission. Inthis case, the second optical signal transmitted from the second opticaltransmitter 82 is transmitted through the second optical reflectivityvariable mirror 52 to the second main optical transmission line 132.

A signal transmission operation of the second optical add-dropmultiplexer will subsequently be described. The second optical inputsignal is transmitted on the second main optical transmission line 132and then reflected by the second optical reflectivity variable mirror 52before the reflected optical signal is then transmitted on the secondmain optical transmission line 132.

A signal drop operation of the second optical add-drop multiplexer willsubsequently be described. The second optical input signal istransmitted on the second main optical transmission line 132 and thendivided into two parts by the second optical coupler 12. One of thedivided second optical input signals is then transmitted through thesecond subordinate optical transmission line 136 to the optical receiver72.

A signal add operation of the second optical add-drop multiplexer willsubsequently be described. If the reflectivity of the second opticalreflectivity variable mirror 52 is set 0%, then the second opticalreflectivity variable mirror 52 is in a transmission state which allowsan optical signal transmission. In this case, a second substituteoptical signal transmitted from the optical transmitter 82 istransmitted through the second optical reflectivity variable mirror 52to the second main optical transmission line 132.

The third optical add-drop multiplexer 3 comprises a third main opticaltransmission line 133 for transmitting the third optical signal, a thirdoptical reflectivity variable mirror 53 provided on the third mainoptical transmission line 133 for reflecting the third optical signal ata controlled reflectivity, and an optical transmitter 83 with an end ofthe third main optical transmission line 133, a third subordinateoptical transmission line 137 connected through a third optical coupler13 to the third main optical transmission line, and a third receiver 73connected with said third subordinate optical transmission line 137. Thethird optical input signal is divided by the third optical coupler 13 sothat one of the divided third optical input signal is transmittedthrough the third subordinate optical transmission line 137 to theoptical receiver 73, whilst the remaining one of the divided thirdoptical input signal is transmitted to the optical reflectivity variablemirror 53 whereby the remaining one of the divided third optical inputsignal is reflected by the third optical reflectivity variable mirror 53at a controlled reflectivity. The third optical reflectivity variablemirror 53 is capable of varying a reflectivity in the range of from 0%to 100%. If the reflectivity of the third optical reflectivity variablemirror 53 is set 0%, then the third optical reflectivity variable mirror53 is in a transmission state which allows an optical signaltransmission. In this case, the third optical signal transmitted fromthe third optical transmitter 83 is transmitted through the thirdoptical reflectivity variable mirror 53 to the third main opticaltransmission line 133.

A signal transmission operation of the third optical add-dropmultiplexer will subsequently be described. The third optical inputsignal is transmitted on the third main optical transmission line 133and then reflected by the third optical reflectivity variable mirror 53before the reflected optical signal is then transmitted on the thirdmain optical transmission line 133.

A signal drop operation of the third optical add-drop multiplexer willsubsequently be described. The third optical input signal is transmittedon the third main optical transmission line 133 and then divided intotwo parts by the third optical coupler 13. One of the divided thirdoptical input signals is then transmitted through the third subordinateoptical transmission line 137 to the optical receiver 73.

A signal add operation of the third optical add-drop multiplexer willsubsequently be described. If the reflectivity of the third opticalreflectivity variable mirror 53 is set 0%, then the third opticalreflectivity variable mirror 53 is in a transmission state which allowsan optical signal transmission. In this case, a third substitute opticalsignal transmitted from the optical transmitter 83 is transmittedthrough the third optical reflectivity variable mirror 53 to the thirdmain optical transmission line 133.

The fourth optical add-drop multiplexer 4 comprises a fourth mainoptical transmission line 134 for transmitting the fourth opticalsignal, a fourth optical reflectivity variable mirror 54 provided on thefourth main optical transmission line 134 of reflecting the fourthoptical signal at a controlled reflectivity, and an optical transmitter84 with an end of the fourth main optical transmission line 134, afourth subordinate optical transmission line 138 connected through afourth optical coupler 14 to the fourth main optical transmission line,and a fourth optical receiver 74 connected with said fourth subordinateoptical transmission line 138. The fourth optical input signal isdivided by the fourth optical coupler 14 so that one of the dividedfourth optical input signal is transmitted through the fourthsubordinate optical transmission line 138 to the optical receiver 74,whilst the remaining one of the divided fourth optical input signal istransmitted to the optical reflectivity variable mirror 54 whereby theremaining one of the divided fourth optical input signal is reflected bythe fourth optical reflectivity variable mirror 54 at a controlledreflectivity. The fourth optical reflectivity variable mirror 54 iscapable of varying a reflectivity in the range of from 0% to 100%. Ifthe reflectivity of the fourth optical reflectivity variable mirror 54is set 0%, then the fourth-optical reflectivity variable mirror 54 is ina transmission state which allows an optical signal transmission. Inthis case, the fourth optical signal transmitted from the fourth opticaltransmitter 84 is transmitted through the fourth optical reflectivityvariable mirror 54 to the fourth main optical transmission line 134.

A signal transmission operation of the fourth optical add-dropmultiplexer will subsequently be described. The fourth optical inputsignal is transmitted on the fourth main optical transmission line 134and then reflected by the fourth optical reflectivity variable mirror 54before the reflected optical signal is then transmitted on the fourthmain optical transmission line 134.

A signal drop operation of the fourth optical add-drop multiplexer willsubsequently be described. The fourth optical input signal istransmitted on the fourth main optical transmission line 134 and thendivided into two parts by the fourth optical coupler 14. One of thedivided fourth optical input signals is then transmitted through thefourth subordinate optical transmission line 138 to the optical receiver74.

A signal add operation of the fourth optical add-drop multiplexer willsubsequently be described. If the reflectivity of the fourth opticalreflectivity variable mirror 54 is set 0%, then the fourth opticalreflectivity variable mirror 54 is in a transmission state which allowsan optical signal transmission. In this case, a fourth substituteoptical signal transmitted from the optical transmitter 84 istransmitted through the fourth optical reflectivity variable mirror 54to the fourth main optical transmission line 134.

First, second, third and fourth output signals are multiplexed by theoptical multiplexer/demultiplexer 410 to form a single output signalwhich is then transmitted through the circulator 60 to the opticaltransmission line 121.

The above novel optical add-drop multiplexers do require no opticalcoupler for signal adding, thereby realizing a low optical loss.

As a modification to this embodiment, it is possible to provide any oneof the above first to fourth optical switches of FIGS. 1 to 4 in thefirst to fourth embodiments, in place of the above optical couplers andthe optical reflectivity variable mirrors.

It is possible that the input and output ports are commonly used or thatthe input and output ports are separated from each other by use of anoptical coupler and an optical isolator or by use of a circulator.

It is also possible to change the number of the wavelength-multiplexingfrom four into, for example, eight, sixteen, thirty two or sixty four.

It is also possible to change the wavelength of the optical signals andalso change a bit rate or a signal rate to 2.5 Gbps, 5 Gbps, 100 Gbps orset a bit-rate free.

The optical multiplexer, the optical demultiplexer or the opticalmultiplexer/demultiplexer may comprise an array waveguide grating, awavelength router having substantially the same grating structure as thearray waveguide grating, or a wavelength MUX coupler havingsubstantially the same grating structure as the array waveguide grating.

Since insertion loss is different among the optical multiplexer, theoptical demultiplexer and the optical multiplexer/demultiplexer, it ispossible to use optical attenuators in individual waveguides for controlof the optical power levels.

It is also possible to control a gain of the erbium doped fiberamplifier gate or control reflectivity of the reflective mirror forcontrol of the optical power levels for every wavelengths separately.

It is furthermore possible to replace the erbium doped fiber by rareearth doped fiber such as tellurium doped fiber, or an aluminum dopedfiber. The length of the rare earth doped fiber and a dopingconcentration thereof may be set in accordance with the requiredspecifications of the optical switch.

The excitation light may have a wavelength of 980 nanometers in order toshorten the wavelength for a remarkable reduction in noise factor of theoptical output signal. In this case, the optical switch is also freefrom any substantive insertion loss and a low or reduced crosstalk.

Thirteenth Embodiment

A thirteenth embodiment according to the present invention will bedescribed in detail with reference to FIG. 15 which is a diagramillustrative of a novel wavelength-multiplexed optical add-dropmultiplexer using four sets of the above novel optical add-dropmultiplexer of FIG. 12.

The novel wavelength-multiplexed optical add-drop multiplexer comprisesa single optical circulator 60 connected with optical transmission lines110, 120 and 121, an optical multiplexer/demultiplexer 410 connectedthrough said optical transmission line 120 to said optical circulator 60and first to fourth optical add-drop multiplexers 1, 2, 3 and 4. Thefirst optical add-drop multiplexer is operable for a signal having awavelength of 1548 nanometers. The second optical add-drop multiplexeris operable for a signal having a wavelength of 1550 nanometers. Thethird optical add-drop multiplexer is operable for a signal having awavelength of 1552 nanometers. The fourth optical add-drop multiplexeris operable for a signal having a wavelength of 1554 nanometers.

The optical input signal having four wavelength compositions of 1548nanometers, 1550 nanometers, 1552 nanometers, and 1554 nanometers istransmitted from the optical transmission line 110 through the opticalcirculator 60 to the optical multiplexer/demultiplexer 410, so that theoptical input signal is wavelength-demultiplexer by the opticalmultiplexer/demultiplexer 410 whereby the optical input signal isdivided into a first signal having a wavelength of 1548 nanometers, asecond signal having a wavelength of 1550 nanometers, a third signalhaving a wavelength of 1552 nanometers, and a fourth signal having awavelength of 1554 nanometers. The first, second, third and fourthoptical signals are inputted into the first, second, third and fourthoptical add-drop multiplexers 1, 2, 3 and 4 respectively.

The first optical add-drop multiplexer 1 comprises a first opticaltransmission line 131 for transmitting an optical input signal, a firsterbium doped fiber 41 provided on said first optical transmission line131, a first wavelength band selective optical reflecting mirror 25provided on said first optical transmission line 131, a first excitationlight source 31 connected to said first wavelength band selectiveoptical reflecting mirror 25, a first optical receiver 71 connectedthrough a first receiver side optical coupler 11 to the first opticaltransmission line 131 and an optical transmitter 81 connected through afirst transmitter side optical coupler 15 to the first signal opticaltransmission line 131. The excitation light source 31 is capable ofemitting an excitation light having a different wavelength from thefirst optical signal. The first wavelength band selective opticalreflecting mirror 25 is capable of selecting a reflecting wavelengthband of the first optical signal to be reflected by the first wavelengthband selective optical reflecting mirror 25. The first optical inputsignal is total-reflected by the first wavelength band selective opticalreflecting mirror 25, whilst the first excitation light emitted from thefirst excitation light source 31 is transmitted through the firstwavelength band selective optical reflecting mirror 25 to the firsterbium doped fiber 41, whereby the first erbium doped fiber 41 isexcited by the first excitation light. The first excited erbium dopedfiber 41 is capable of amplifying the first optical input signal. Theamplified input signal is then total-reflected by the first wavelengthband selective optical reflecting mirror 25. The reflected input signalis then transmitted again through the first erbium doped fiber 41,whereby the reflected signal is further amplified. The further amplifiedoptical signal is transmitted through the first output signaltransmission line 131.

A signal transmission operation of the above first optical add-dropmultiplexer will subsequently be described. The first excitation lightis emitted from the first excitation light source 31 and then suppliedto the first erbium doped fiber 41, whereby the first erbium doped fiber41 is excited. The optical input signal is transmitted through the firsterbium doped fiber 41 and amplified by the excited first erbium dopedfiber 41. The amplified optical input signal is total-reflected by thefirst wavelength band selective optical reflecting mirror 25. Thereflected optical signal is then transmitted again through the firsterbium doped fiber 41, whereby the reflected signal is furtheramplified. The further amplified optical signal is transmitted throughthe first signal transmission line 131 to the opticalmultiplexer/demultiplexer 410.

A signal drop operation of the above first optical add-drop multiplexerwill subsequently be described. The first optical input signal isdivided by the first receiver side optical coupler 11 into two parts,one of which is transmitted to the first optical receiver 71.

A signal add operation of the above first optical add-drop multiplexerwill subsequently be described. In this case, no excitation light isemitted from the first excitation light source 31, for which reason thefirst erbium doped fiber 41 receives no excitation light and isunexcited, whereby the first input optical signal is absorbed by thefirst erbium doped fiber 41. No optical signal is outputted from thefirst signal transmission line 131. On the other hand, the first opticaltransmitter 81 emits a first substitute optical signal which is thentransmitted through the first transmitter side optical coupler 15 on thefirst optical transmission line 131, whereby the first substituteoptical signal is outputted from the first optical transmission line131.

The second optical add-drop multiplexer 2 comprises a second opticaltransmission line 132 for transmitting an optical input signal, a seconderbium doped fiber 42 provided on said second optical transmission line132, a second wavelength band selective optical reflecting mirror 26provided on said second optical transmission line 132, a secondexcitation light source 32 connected to said second wavelength bandselective optical reflecting mirror 26, a second optical receiver 72connected through a second receiver side optical coupler 12 to thesecond optical transmission line 132 and an optical transmitter 82connected through a second transmitter side optical coupler 16 to thesecond signal optical transmission line 132. The excitation light source32 is capable of emitting an excitation light having a differentwavelength from the second optical signal. The second wavelength bandselective optical reflecting mirror 26 is capable of selecting areflecting wavelength band of the second optical signal to be reflectedby the second wavelength band selective optical reflecting mirror 26.The second optical input signal is total-reflected by the secondwavelength band selective optical reflecting mirror 26, whilst thesecond excitation light emitted from the second excitation light source32 is transmitted through the second wavelength band selective opticalreflecting mirror 26 to the second erbium doped fiber 42, whereby thesecond erbium doped fiber 42 is excited by the second excitation light.The second excited erbium doped fiber 42 is capable of amplifying thesecond optical input signal. The amplified input signal is thentotal-reflected by the second wavelength band selective opticalreflecting mirror 26. The reflected input signal is then transmittedagain through the second erbium doped fiber 42, whereby the reflectedsignal is further amplified. The further amplified optical signal istransmitted through the second output signal transmission line 132.

A signal transmission operation of the above second optical add-dropmultiplexer will subsequently be described. The second excitation lightis emitted from the second excitation light source 32 and then suppliedto the second erbium doped fiber 42, whereby the second erbium dopedfiber 42 is excited. The optical input signal is transmitted through thesecond erbium doped fiber 42 and amplified by the excited second erbiumdoped fiber 42. The amplified optical input signal is total-reflected bythe second wavelength band selective optical reflecting mirror 26. Thereflected optical signal is then transmitted again through the seconderbium doped fiber 42, whereby the reflected signal is furtheramplified. The further amplified optical signal is transmitted throughthe second signal transmission line 132 to the opticalmultiplexer/demultiplexer 420.

A signal drop operation of the above second optical add-drop multiplexerwill subsequently be described. The second optical input signal isdivided by the second receiver side optical coupler 12 into two parts,one of which is transmitted to the second optical receiver 72.

A signal add operation of the above second optical add-drop multiplexerwill subsequently be described. In this case, no excitation light isemitted from the second excitation light source 32, for which reason thesecond erbium doped fiber 42 receives no excitation light and isunexcited, whereby the second input optical signal is absorbed by thesecond erbium doped fiber 42. No optical signal is outputted from thesecond signal transmission line 132. On the other hand, the secondoptical transmitter 82 emits a second substitute optical signal which isthen transmitted through the second transmitter side optical coupler 16on the second optical transmission line 132, whereby the secondsubstitute optical signal is outputted from the second opticaltransmission line 132.

The third optical add-drop multiplexer 3 comprises a third opticaltransmission line 133 for transmitting an optical input signal, a thirderbium doped fiber 43 provided on said third optical transmission line133, a third wavelength band selective optical reflecting mirror 27provided on said third optical transmission line 133, a third excitationlight source 33 connected to said third wavelength band selectiveoptical reflecting mirror 27, a third optical receiver 73 connectedthrough a third receiver side optical coupler 13 to the third opticaltransmission line 133 and an optical transmitter 83 connected through athird transmitter side optical coupler 17 to the third signal opticaltransmission line 133. The excitation light source 33 is capable ofemitting an excitation light having a different wavelength from thethird optical signal. The third wavelength band selective opticalreflecting mirror 27 is capable of selecting a reflecting wavelengthband of the third optical signal to be reflected by the third wavelengthband selective optical reflecting mirror 27. The third optical inputsignal is total-reflected by the third wavelength band selective opticalreflecting mirror 27, whilst the third excitation light emitted from thethird excitation light source 33 is transmitted through the thirdwavelength band selective optical reflecting mirror 27 to the thirderbium doped fiber 43, whereby the third erbium doped fiber 43 isexcited by the third excitation light. The third excited erbium dopedfiber 43 is capable of amplifying the third optical input signal. Theamplified input signal is then total-reflected by the third wavelengthband selective optical reflecting mirror 27. The reflected input signalis then transmitted again through the third erbium doped fiber 43,whereby the reflected signal is further amplified. The further amplifiedoptical signal is transmitted through the third output signaltransmission line 133.

A signal transmission operation of the above third optical add-dropmultiplexer will subsequently be described. The third excitation lightis emitted from the third excitation light source 33 and then suppliedto the third erbium doped fiber 43, whereby the third erbium doped fiber43 is excited. The optical input signal is transmitted through the thirderbium doped fiber 43 and amplified by the excited third erbium dopedfiber 43. The amplified optical input signal is total-reflected by thethird wavelength band selective optical reflecting mirror 27. Thereflected optical signal is then transmitted again through the thirderbium doped fiber 43, whereby the reflected signal is furtheramplified. The further amplified optical signal is transmitted throughthe third signal transmission line 133 to the opticalmultiplexer/demultiplexer 430.

A signal drop operation of the above third optical add-drop multiplexerwill subsequently be described. The third optical input signal isdivided by the third receiver side optical coupler 13 into two parts,one of which is transmitted to the third optical receiver 73.

A signal add operation of the above third optical add-drop multiplexerwill subsequently be described. In this case, no excitation light isemitted from the third excitation light source 33, for which reason thethird erbium doped fiber 43 receives no excitation light and isunexcited, whereby the third input optical signal is absorbed by thethird erbium doped fiber 43. No optical signal is outputted from thethird signal transmission line 133. On the other hand, the third opticaltransmitter 83 emits a third substitute optical signal which is thentransmitted through the third transmitter side optical coupler 17 on thethird optical transmission line 133, whereby the third substituteoptical signal is outputted from the third optical transmission line133.

The fourth optical add-drop multiplexer 4 comprises a fourth opticaltransmission line 134 for transmitting an optical input signal, a fourtherbium doped fiber 44 provided on said fourth optical transmission line134, a fourth wavelength band selective optical reflecting mirror 28provided on said fourth optical transmission line 134, a fourthexcitation light source 34 connected to said fourth wavelength bandselective optical reflecting mirror 28, a fourth optical receiver 74connected through a fourth receiver side optical coupler 14 to thefourth optical transmission line 134 and an optical transmitter 84connected through a fourth transmitter side optical coupler 18 to thefourth signal optical transmission line 134. The excitation light source34 is capable of emitting an excitation light having a differentwavelength from the fourth optical signal. The fourth wavelength bandselective optical reflecting mirror 28 is capable of selecting areflecting wavelength band of the fourth optical signal to be reflectedby the fourth wavelength band selective optical reflecting mirror 28.The fourth optical input signal is total-reflected by the fourthwavelength band selective optical reflecting mirror 28, whilst thefourth excitation light emitted from the fourth excitation light source34 is transmitted through the fourth wavelength band selective opticalreflecting mirror 28 to the fourth erbium doped fiber 44, whereby thefourth erbium doped fiber 44 is excited by the fourth excitation light.The fourth excited erbium doped fiber 44 is capable of amplifying thefourth optical input signal. The amplified input signal is thentotal-reflected by the fourth wavelength band selective opticalreflecting mirror 28. The reflected input signal is then transmittedagain through the fourth erbium doped fiber 44, whereby the reflectedsignal is further amplified. The further amplified optical signal istransmitted through the fourth output signal transmission line 134.

A signal transmission operation of the above fourth optical add-dropmultiplexer will subsequently be described. The fourth excitation lightis emitted from the fourth excitation light source 34 and then suppliedto the fourth erbium doped fiber 44, whereby the fourth erbium dopedfiber 44 is excited. The optical input signal is transmitted through thefourth erbium doped fiber 44 and amplified by the excited fourth erbiumdoped fiber 44. The amplified optical input signal is total-reflected bythe fourth wavelength band selective optical reflecting mirror 28. Thereflected optical signal is then transmitted again through the fourtherbium doped fiber 44, whereby the reflected signal is furtheramplified. The further amplified optical signal is transmitted throughthe fourth signal transmission line 134 to the opticalmultiplexer/demultiplexer 440.

A signal drop operation of the above fourth optical add-drop multiplexerwill subsequently be described. The fourth optical input signal isdivided by the fourth receiver side optical coupler 14 into two parts,one of which is transmitted to the fourth optical receiver 74.

A signal add operation of the above fourth optical add-drop multiplexerwill subsequently be described. In this case, no excitation light isemitted from the fourth excitation light source 34, for which reason thefourth erbium doped fiber 44 receives no excitation light and isunexcited, whereby the fourth input optical signal is absorbed by thefourth erbium doped fiber 44. No optical signal is outputted from thefourth signal transmission line 134. On the other hand, the fourthoptical transmitter 84 emits a fourth substitute optical signal which isthen transmitted through the fourth transmitter side optical coupler 18on the fourth optical transmission line 134, whereby the fourthsubstitute optical signal is outputted from the fourth opticaltransmission line 134.

First, second, third and fourth output signals are multiplexed by theoptical multiplexer/demultiplexer 410 to form a single output signalwhich is then transmitted through the circulator 60 to the opticaltransmission line 121.

The above novel optical add-drop multiplexer is capable of reducing aninsertion loss and also reducing the number of the required opticalcouplers.

It is also possible to integrate the wavelength band selective opticalreflecting mirrors and the excitation light sources.

It is possible that the input and output ports are commonly used or thatthe input and output ports are separated from each other by use of anoptical coupler and an optical isolator or by use of a circulator.

It is also possible to change the number of the wavelength-multiplexingfrom four into, for example, eight, sixteen, thirty two or sixty four.

It is also possible to change the wavelength of the optical signals andalso change a bit rate or a signal rate to 2.5 Gbps, 5 Gbps, 100 Gbps orset a bit-rate free.

The optical multiplexer, the optical demultiplexer or the opticalmultiplexer/demultiplexer may comprise an array waveguide grating, awavelength router having substantially the same grating structure as thearray waveguide grating, or a wavelength MUX coupler havingsubstantially the same grating structure as the array waveguide grating.

Since insertion loss is different among the optical multiplexer, theoptical demultiplexer and the optical multiplexer/demultiplexer, it ispossible to use optical attenuators in individual waveguides for controlof the optical power levels.

It is also possible to control a gain of the erbium doped fiberamplifier gate or control reflectivity of the reflective mirror forcontrol of the optical power levels for every wavelengths separately.

It is furthermore possible to replace the erbium doped fiber by rareearth doped fiber such as tellurium doped fiber and a dopingconcentration thereof may be set in accordance with the requiredspecifications of the optical switch.

The excitation light may have a wavelength of 980 nanometers in order toshorten the wavelength for a remarkable reduction in noise factor of theoptical output signal. In this case, the optical switch is also freefrom any substantive insertion loss and a low or reduced crosstalk.

Fourteenth Embodiment

A fourteenth embodiment according to the present invention will bedescribed in detail with reference to FIG. 16 which is a diagramillustrative of a novel wavelength-multiplexed optical add-dropmultiplexer having four looped optical transmission paths.

The novel wavelength-multiplexed optical add-drop multiplexer comprisesan optical multiplexer/demultiplexer 410 having an input port 110 and anoutput port 120, and first to fourth optical transmission lines 131,132, 133 and 134 connected to the optical multiplexer/demultiplexer 410.The first optical transmission line 131 is provided for transmitting asignal having a wavelength of 1530 nanometers. The second opticaltransmission line 132 is provided for transmitting a signal having awavelength of 1540 nanometers. The third optical transmission line 133is provided for transmitting a signal having a wavelength of 1550nanometers. The fourth optical transmission line is provided fortransmitting a signal having a wavelength of 1560 nanometers.

The optical input signal having four wavelength compositions of 1530nanometers, 1540 nanometers, 1550 nanometers, and 1560 nanometers istransmitted from the optical transmission line 110 to the opticalmultiplexer/demultiplexer 410, so that the optical input signal iswavelength-demultiplexed by the optical multiplexer/demultiplexer 410whereby the optical input signal is divided into a first signal having awavelength of 1530 nanometers, a second signal having a wavelength of1540 nanometers, a third signal having a wavelength of 1550 nanometers,and a fourth signal having a wavelength of 1560 nanometers. The first,second, third and fourth optical signals are inputted into the first,second, third and fourth optical transmission lines 131, 132, 133 and134 respectively.

The first optical transmission line 131 has a first receiver sideoptical coupler 31 which is connected to a first optical receiver 71,and a first transmitter side optical coupler 44 which is connected to afirst optical transmitter 84. The first optical transmission line 131also has a first optical multiplexer/demultiplexer 151.

FIG. 17 is a diagram illustrative of the first opticalmultiplexer/demultiplexer 151 used in the wavelength-multiplexed opticaladd-drop multiplexer of FIG. 16. The first opticalmultiplexer/demultiplexer 151 performs a wavelength demultiplexing so asto divide the first optical input signal into two different wavelengthoptical signals having 1.55 micrometers and 1.54 micrometers which areoutputted from two output ports. The first opticalmultiplexer/demultiplexer 151 is used in place of the optical coupler sothat the wavelength different two optical signals has a total opticalpower which is higher than the first optical signal, thereby to solve aproblem with remarkable optical power loss caused when the signal istransmitted through a plurality of optical couplers.

The second optical transmission line 132 has a second receiver sideoptical coupler 32 which is connected to a second optical receiver 72,and a second transmitter side optical coupler 41 which is connected to asecond optical transmitter 81. The second optical transmission line 132also has a second optical multiplexer/demultiplexer 152. The firstoptical multiplexer/demultiplexer 151 is also connected through a seriesconnection of a first optical amplifier 51 and a first isolator 91 tothe second optical multiplexer/demultiplexer 152. One of thewavelength-demultiplexed optical signals is transmitted from the firstoptical multiplexer/demultiplexer 151 through the first opticalamplifier 51 and the first isolator 91 to the second opticalmultiplexer/demultiplexer 152.

The second optical multiplexer/demultiplexer 152 performs a wavelengthdemultiplexing so as to divide the first optical input signal into twodifferent wavelength optical signals which are outputted from two outputports. The second optical multiplexer/demultiplexer 152 is used in placeof the optical coupler so that the wavelength different two opticalsignals has a total optical power which is higher than the secondoptical signal, thereby to solve a problem with remarkable optical powerloss caused when the signal is transmitted through a plurality ofoptical couplers.

The third optical transmission line 133 has a third receiver sideoptical coupler 33 which is connected to a third optical receiver 73,and a third transmitter side optical coupler 42 which is connected to athird optical transmitter 82. The third optical transmission line 133also has a third optical multiplexer/demultiplexer 153. The secondoptical multiplexer/demultiplexer 152 is also connected through a seriesconnection of a second optical amplifier 52 and a second isolator 92 tothe third optical multiplexer/demultiplexer 153. One of thewavelength-demultiplexed optical signals is transmitted from the secondoptical multiplexer/demultiplexer 152 through the second opticalamplifier 52 and the second isolator 92 to the third opticalmultiplexer/demultiplexer 153.

Third optical multiplexer/demultiplexer 153 performs a wavelengthdemultiplexing so as to divide the first optical input signal into twodifferent wavelength optical signals which are outputted from two outputports. The third optical multiplexer/demultiplexer 153 is used in placeof the optical coupler so that the wavelength different two opticalsignals has a total optical power which is higher than the third opticalsignal, thereby to solve a problem with remarkable optical power losscaused when the signal is transmitted through a plurality of opticalcouplers.

The fourth optical transmission line 134 has a fourth receiver sideoptical coupler 34 which is connected to a fourth optical receiver 73,and a fourth transmitter side optical coupler 43 which is connected to afourth optical transmitter 82. The fourth optical transmission line 134also has a fourth optical multiplexer/demultiplexer 154. The thirdoptical multiplexer/demultiplexer 153 is also connected through a seriesconnection of a third optical amplifier 52 and a third isolator 93 tothe fourth optical multiplexer/demultiplexer 154. One of thewavelength-demultiplexed optical signals is transmitted from the thirdoptical multiplexer/demultiplexer 153 through the third opticalamplifier 52 and the third isolator 93 to the fourth opticalmultiplexer/demultiplexer 154.

The fourth optical multiplexer/demultiplexer 154 performs a wavelengthdemultiplexing so as to divide the first optical input signal into twodifferent wavelength optical signals which are outputted from two outputports. The fourth optical multiplexer/demultiplexer 154 is used in placeof the optical coupler so that the wavelength different two opticalsignals has a total optical power which is higher than the fourthoptical signal, thereby to solve a problem with remarkable optical powerloss caused when the signal is transmitted through a plurality ofoptical couplers.

The fourth optical multiplexer/demultiplexer 154 is also connectedthrough a series connection of a fifth optical amplifier 54 and a fifthoptical attenuator 94 to the first optical multiplexer/demultiplexer151.

The above wavelength-multiplexed optical add-drop multiplexer performssignal transmission operation, signal drop operation and signal addoperation.

The signal transmission operation of the wavelength-multiplexed opticaladd-drop multiplexer will be described. The first input signal istransmitted through the first optical multiplexer/demultiplexer 151 tothe first optical amplifier 51, whereby the signal is amplified by thefirst optical amplifier 51. The amplified signal is then transmittedthrough the first optical isolator 91 to the second opticalmultiplexer/demultiplexer 152. Since the second opticalmultiplexer/demultiplexer 152 has a multiplexing function, the amplifiedsignal is transmitted through the second optical transmission line 132to the optical multiplexer/demultiplexer 141, whereby the signal ismultiplexed with other signal to output an output signal from the outputport 120. The second input signal is transmitted through the secondoptical multiplexer/demultiplexer 152 to the second optical amplifier52, whereby the signal is amplified by the second optical amplifier 52.The amplified signal is then transmitted through the second opticalisolator 92 to the third optical multiplexer/demultiplexer 153. Sincethe third optical multiplexer/demultiplexer 153 has a multiplexingfunction, the amplified signal is transmitted through the third opticaltransmission line 133 to the optical multiplexer/demultiplexer 141,whereby the signal is multiplexed with other signal to output an outputsignal from the output port 120. The third input signal is transmittedthrough the third optical multiplexer/demultiplexer 153 to the thirdoptical amplifier 53, whereby the signal is amplified by the thirdoptical amplifier 53. The amplified signal is then transmitted throughthe third optical isolator 93 to the fourth opticalmultiplexer/demultiplexer 154. Since the fourth opticalmultiplexer/demultiplexer 154 has a multiplexing function, the amplifiedsignal is transmitted through the fourth optical transmission line 134to the optical multiplexer/demultiplexer 141, whereby the signal ismultiplexed with other signal to output an output signal from the outputport 120. The fourth input signal is transmitted through the fourthoptical multiplexer/demultiplexer 154 to the fourth optical amplifier54, whereby the signal is amplified by the fourth optical amplifier 54.The amplified signal is then transmitted through the fourth opticalisolator 94 to the first optical multiplexer/demultiplexer 151. Sincethe first optical multiplexer/demultiplexer 151 has a multiplexingfunction, the amplified signal is transmitted through the first opticaltransmission line 131 to the optical multiplexer/demultiplexer 141,whereby the signal is multiplexed with other signal to output an outputsignal from the output port 120.

The signal drop operation of the wavelength-multiplexed optical add-dropmultiplexer will be described. The first input signal is transmittedfrom the first optical transmission line 131 through the first receiverside optical coupler 31 into the first optical receiver 71. The secondinput signal is transmitted from the second optical transmission line132 through the second receiver side optical coupler 32 into the secondoptical receiver 72. The third input signal is transmitted from thethird optical transmission line 133 through the third receiver sideoptical coupler 33 into the third optical receiver 73. The fourth inputsignal is transmitted from the fourth optical transmission line 134through the fourth receiver side optical coupler 34 into the fourthoptical receiver 74.

The signal add operation of the wavelength-multiplexed optical add-dropmultiplexer will be described. The first optical amplifier 51 turns OFF,whereby the transmission of the first optical signal through the firstoptical transmission line 131 and the first opticalmultiplexer/demultiplexer 151 is discontinued by the first opticalamplifier 51, whereby no signal is transmitted through the secondoptical multiplexer/demultiplexer 152 to the second optical transmissionline 132. On the other hand, a first substitute signal is transmittedfrom the second optical transmitter 81 so that the first substitutesignal is then transmitted through the second optical transmission line132 to the optical multiplexer/demultiplexer 141, whereby the signal ismultiplexed with other signal to output an output signal from the outputport 120. The second optical amplifier 52 turns OFF, whereby thetransmission of the second optical signal through the second opticaltransmission line 132 and the second optical multiplexer/demultiplexer152 is discontinued by the second optical amplifier 52, whereby nosignal is transmitted through the third opticalmultiplexer/demultiplexer 153 to the third optical transmission line133. On the other hand, a second substitute signal is transmitted fromthe third optical transmitter 82 so that the second substitute signal isthen transmitted through the third optical transmission line 133 to theoptical multiplexer/demultiplexer 141, whereby the signal is multiplexedwith other signal to output an output signal from the output port 120.The third optical amplifier 53 turns OFF, whereby the transmission ofthe third optical signal through the third optical transmission line 133and the third optical multiplexer/demultiplexer 153 is discontinued bythe third optical amplifier 53, whereby no signal is transmitted throughthe fourth optical multiplexer/demultiplexer 154 to the fourth opticaltransmission line 134. On the other hand, a third substitute signal istransmitted from the fourth optical transmitter 83 so that the thirdsubstitute signal is then transmitted through the fourth opticaltransmission line 134 to the optical multiplexer/demultiplexer 141,whereby the signal is multiplexed with other signal to output an outputsignal from the output port 120. The fourth optical amplifier 54 turnsOFF, whereby the transmission of the fourth optical signal through thefourth optical transmission line 134 and the fourth opticalmultiplexer/demultiplexer 154 is discontinued by the fourth opticalamplifier 54, whereby no signal is transmitted through the first opticalmultiplexer/demultiplexer 151 to the first optical transmission line131. On the other hand, a fourth substitute signal is transmitted fromthe first optical transmitter 84 so that the fourth substitute signal isthen transmitted through the first optical transmission line 131 to theoptical multiplexer/demultiplexer 141, whereby the signal is multiplexedwith other signal to output an output signal from the output port 120.

The use of the optical wavelength-multiplexer/demultiplexer to serve asthe same function as the optical coupler reduces the optical power lossby not less than 5 dB as compared to the 1:1 optical coupler.

Fifteenth Embodiment

A fifteenth embodiment according to the present invention will bedescribed in detail with reference to FIG. 18 which is a diagramillustrative of a novel wavelength-multiplexed optical add-dropmultiplexer having four looped optical transmission paths.

The novel wavelength-multiplexed optical add-drop multiplexer comprisesan optical multiplexer/demultiplexer 410 having an input port 110 and anoutput port 120, and first to fourth optical transmission lines 131,132, 133 and 134 connected to the optical multiplexer/demultiplexer 410.The first optical transmission line 131 is provided for transmitting asignal having a wavelength of 1530 nanometers. The second opticaltransmission line 132 is provided for transmitting a signal having awavelength of 1540 nanometers. The third optical transmission line 133is provided for transmitting a signal having a wavelength of 1550nanometers. The fourth optical transmission line is provided fortransmitting a signal having a wavelength of 1560 nanometers.

The optical input signal having four wavelength composition of 1530nanometers, 1540 nanometers, 1550 nanometers, and 1560 nanometers istransmitted from the optical transmission line 110 to the opticalmultiplexer/demultiplexer 410, so that the optical input signal iswavelength-demultiplexed by the optical multiplexer/demultiplexer 410whereby the optical input signal is divided into a first signal having awavelength of 1530 nanometers, a second signal having a wavelength of1540 nanometers, a third signal having a wavelength of 1550 nanometers,and a fourth signal having a wavelength of 1560 nanometers. The first,second, third and fourth optical signals are inputted into the first,second, third and fourth optical transmission lines 131, 132, 133 and134 respectively.

The first optical transmission line 131 has a first receiver sideoptical coupler 31 which is connected to a first optical receiver 71,and a first transmitter side optical coupler 44 which is connected to afirst optical transmitter 84. The first optical transmission line 131also has a first optical circulator 61.

The second optical transmission line 132 has a second receiver sideoptical coupler 32 which is connected to a second optical receiver 72,and a second transmitter side optical coupler 41 which is connected to asecond optical transmitter 81. The second optical transmission line 132also has a second optical circulator 62. The first optical circulator 61is also connected through a first optical amplifier 51 to the secondoptical circulator 62. One of the wavelength-demultiplexed opticalsignals is transmitted from the first optical circulator 61 through thefirst optical amplifier 51 to the second optical circulator 62.

The third optical transmission line 133 has a third receiver sideoptical coupler 33 which is connected to a third optical receiver 73,and a third transmitter side optical coupler 42 which is connected to athird optical transmitter 82. The third optical transmission line 133also has a third optical circulator 63. The second optical circulator 62is also connected through a second optical amplifier 52 to the thirdoptical circulator 63. One of the wavelength-demultiplexed opticalsignals is transmitted from the second optical circulator 62 through thesecond optical amplifier 52 to the third optical circulator 63.

The fourth optical transmission line 134 has a fourth receiver sideoptical coupler 34 which is connected to a fourth optical receiver 73,and a fourth transmitter side optical coupler 43 which is connected to afourth optical transmitter 82. The fourth optical transmission line 134also has a fourth optical circulator 64. The third optical circulator 63is also connected through a third optical amplifier 52 to the fourthoptical circulator 64. One of the wavelength-demultiplexed opticalsignals is transmitted from the third optical circulator 63 through thethird optical amplifier 52 to the fourth optical circulator 64.

The fourth optical circulator 64 is also connected through a seriesconnection of a fifth optical amplifier 54 and a fifth opticalattenuator 94 to the first optical circulator 61.

The above wavelength-multiplexed optical add-drop multiplexer performssignal transmission operation, signal drop operation and signal addoperation.

The signal transmission operation of the wavelength-multiplexed opticaladd-drop multiplexer will be described. The first input signal istransmitted through the first optical circulator 61 to the first opticalamplifier 51, whereby the signal is amplified by the first opticalamplifier 51. The amplified signal is then transmitted to the secondoptical circulator 62. The amplified signal is transmitted through thesecond optical transmission line 132 to the opticalmultiplexer/demultiplexer 141, whereby the signal is multiplexed withother signal to output an output signal from the output port 120. Thesecond input signal is transmitted through the second optical circulator62 to the second optical amplifier 52, whereby the signal is amplifiedby the second optical amplifier 52. The amplified signal is thentransmitted to the third optical circulator 63. The amplified signal istransmitted through the third optical transmission line 133 to theoptical multiplexer/demultiplexer 141, whereby the signal is multiplexedwith other signal to output an output signal from the output port 120.The third input signal is transmitted through the third opticalcirculator 63 to the third optical amplifier 53, whereby the signal isamplified by the third optical amplifier 53. The amplified signal isthen transmitted to the fourth optical circulator 64. The amplifiedsignal is transmitted through the fourth optical transmission line 134to the optical multiplexer/demultiplexer 141, whereby the signal ismultiplexed with other signal to output an output signal from the outputport 120. The fourth input signal is transmitted through the fourthoptical circulator 64 to the fourth optical amplifier 54, whereby thesignal is amplified by the fourth optical amplifier 54. The amplifiedsignal is then transmitted to the first optical circulator 61. Theamplified signal is transmitted through the first optical transmissionline 131 to the optical multiplexer/demultiplexer 141, whereby thesignal is multiplexed with other signal to output an output signal fromthe output port 120.

The signal drop operation of the wavelength-multiplexed optical add-dropmultiplexer will be described. The first input signal is transmittedfrom the first optical transmission line 131 through the first receiverside optical coupler 31 into the first optical receiver 71. The secondinput signal is transmitted from the second optical transmission line132 through the second receiver side optical coupler 32 into the secondoptical receiver 72. The third input signal is transmitted from thethird optical transmission line 133 through the third receiver sideoptical coupler 33 into the third optical receiver 73. The fourth inputsignal is transmitted from the fourth optical transmission line 134through the fourth receiver side optical coupler 34 into the fourthoptical receiver 74.

The signal add operation of the wavelength-multiplexed optical add-dropmultiplexer will be described. The first optical amplifier 51 turns OFF,whereby the transmission of the first optical signal through the firstoptical transmission line 131 and the first optical circulator 61 isdiscontinued by the first optical amplifier 51, whereby no signal istransmitted through the second optical circulator 62 to the secondoptical transmission line 132. On the other hand, a first substitutesignal is transmitted from the second optical transmitter 81 so that thefirst substitute signal is then transmitted through the second opticaltransmission line 132 to the optical multiplexer/demultiplexer 141,whereby the signal is multiplexed with other signal to output an outputsignal from the output port 120. The second optical amplifier 52 turnsOFF, whereby the transmission of the second optical signal through thesecond optical transmission line 132 and the second optical circulator62 is discontinued by the second optical amplifier 52, whereby no signalis transmitted through the third optical circulator 63 to the thirdoptical transmission line 133. On the other hand, a second substitutesignal is transmitted from the third optical transmitter 82 so that thesecond substitute signal is then transmitted through the third opticaltransmission line 133 to the optical multiplexer/demultiplexer 141,whereby the signal is multiplexed with other signal to output an outputsignal from the output port 120. The third optical amplifier 53 turnsOFF, whereby the transmission of the third optical signal through thethird optical transmission line 133 and the third optical circulator 63is discontinued by the third optical amplifier 53, whereby no signal istransmitted through the fourth optical circulator 64 to the fourthoptical transmission line 134. On the other hand, a third substitutesignal is transmitted from the fourth optical transmitter 83 so that thethird substitute signal is then transmitted through the fourth opticaltransmission line 134 to the optical multiplexer/demultiplexer 141,whereby the signal is multiplexed with other signal to output an outputsignal from the output port 120. The fourth optical amplifier 54 turnsOFF, whereby the transmission of the fourth optical signal through thefourth optical transmission line 134 and the fourth optical circulator64 is discontinued by the fourth optical amplifier 54, whereby no signalis transmitted through the first optical circulator 61 to the firstoptical transmission line 131. On the other hand, a fourth substitutesignal is transmitted from the first optical transmitter 84 so that thefourth substitute signal is then transmitted through the first opticaltransmission line 131 to the optical multiplexer/demultiplexer 141,whereby the signal is multiplexed with other signal to output an outputsignal from the output port 120.

The use of the optical isolator to serve as the same function as theoptical coupler reduces the optical power loss by not less than 5 dB ascompared to the 1:1 optical coupler.

Sixteenth Embodiment

A sixteenth embodiment according to the present invention will bedescribed in detail with reference to FIG. 19 which is a diagramillustrative of a novel wavelength-multiplexed optical amplifier havingfour looped optical transmission paths.

The novel wavelength-multiplexed optical amplifier is structurallydifferent from the above wavelength-multiplexed optical add-dropmultiplexer of FIG. 16 in view of no provision of optical receivers andoptical transmitters.

The novel wavelength-multiplexed optical amplifier comprises an opticalmultiplexer/demultiplexer 410 having an input port 110 and an outputport 120, and first to fourth optical transmission lines 131, 132, 133and 134 connected to the optical multiplexer/demultiplexer 410. Thefirst optical transmission line 131 is provided for transmitting asignal having a wavelength of 1530 nanometers. The second opticaltransmission line 132 is provided for transmitting a signal having awavelength of 1540 nanometers. The third optical transmission line 133is provided for transmitting a signal having a wavelength of 1550nanometers. The fourth optical transmission line is provided fortransmitting a signal having a wavelength of 1560 nanometers.

The optical input signal having four wavelength composition of 1530nanometers, 1540 nanometers, 1550 nanometers, and 1560 nanometers istransmitted from the optical transmission line 110 to the opticalmultiplexer/demultiplexer 410, so that the optical input signal iswavelength-demultiplexed by the optical multiplexer/demultiplexer 410whereby the optical input signal is divided into a first signal having awavelength of 1530 nanometers, a second signal having a wavelength of1540 nanometers, a third signal having a wavelength of 1550 nanometers,and a fourth signal having a wavelength of 1560 nanometers. The first,second, third and fourth optical signals are inputted into the first,second, third and fourth optical transmission lines 131, 132, 133 and134 respectively.

The first optical transmission line 131 has a first opticalmultiplexer/demultiplexer 151.

The first optical multiplexer/demultiplexer 151 performs a wavelengthdemultiplexing so as to divide the first optical input signal into twodifferent wavelength optical signals having 1.55 micrometers and 1.54micrometers which are outputted from two output ports. The first opticalmultiplexer/demultiplexer 151 is used in place of the optical coupler sothat the wavelength different two optical signals has a total opticalpower which is higher than the first optical signal, thereby to solve aproblem with remarkable optical power loss caused when the signal istransmitted through a plurality of optical couplers.

The second optical transmission line 132 has a second opticalmultiplexer/demultiplexer 152. The first opticalmultiplexer/demultiplexer 151 is also connected through a seriesconnection of a first optical amplifier 55 and a first isolator 91 tothe second optical multiplexer/demultiplexer 152. One of thewavelength-demultiplexed optical signals is transmitted from the firstoptical multiplexer/demultiplexer 151 through the first opticalamplifier 55 and the first isolator 91 to the second opticalmultiplexer/demultiplexer 152.

The second optical multiplexer/demultiplexer 152 performs a wavelengthdemultiplexing so as to divide the first optical input signal into twodifferent wavelength optical signals which are outputted from two outputports. The second optical multiplexer/demultiplexer 152 is used in placeof the optical coupler so that the wavelength different two opticalsignals has a total optical power which is higher than the secondoptical signal, thereby to solve a problem with remarkable optical powerloss caused when the signal is transmitted through a plurality ofoptical couplers.

The third optical transmission line 133 has a third opticalmultiplexer/demultiplexer 153. The second opticalmultiplexer/demultiplexer 152 is also connected through a seriesconnection of a second optical amplifier 56 and a second isolator 92 tothe third optical multiplexer/demultiplexer 153. One of thewavelength-demultiplexed optical signals is transmitted from the secondoptical multiplexer/demultiplexer 152 through the second opticalamplifier 56 and the second isolator 92 to the third opticalmultiplexer/demultiplexer 153.

The third optical multiplexer/demultiplexer 153 performs a wavelengthdemultiplexing so as to divide the first optical input signal into twodifferent wavelength optical signals which are outputted from two outputports. The third optical multiplexer/demultiplexer 153 is used in placeof the optical coupler so that the wavelength different two opticalsignals has a total optical power which is higher than the third opticalsignal, thereby to solve a problem with remarkable optical power losscaused when the signal is transmitted through a plurality of opticalcouplers.

The fourth optical transmission line 134 has a fourth opticalmultiplexer/demultiplexer 154. The third opticalmultiplexer/demultiplexer 153 is also connected through a seriesconnection of a third optical amplifier 56 and a third isolator 93 tothe fourth optical multiplexer/demultiplexer 154. One of thewavelength-demultiplexed optical signals is transmitted from the thirdoptical multiplexer/demultiplexer 153 through the third opticalamplifier 56 and the third isolator 93 to the fourth opticalmultiplexer/demultiplexer 154.

The fourth optical multiplexer/demultiplexer 154 performs a wavelengthdemultiplexing so as to divide the first optical input signal into twodifferent wavelength optical signals which are outputted from two outputports. The fourth optical multiplexer/demultiplexer 154 is used in placeof the optical coupler so that the wavelength different two opticalsignals has a total optical power which is higher than the fourthoptical signal, thereby to solve a problem with remarkable optical powerloss caused when the signal is transmitted through a plurality ofoptical couplers.

The fourth optical multiplexer/demultiplexer 154 is also connectedthrough a series connection of a fifth optical amplifier 58 and a fifthoptical attenuator 94 to the first optical multiplexer/demultiplexer151.

The above wavelength-multiplexed optical amplifier performs signaltransmission operation.

The signal transmission operation of the wavelength-multiplexer opticalamplifier will described. The first input signal is transmitted throughthe first optical multiplexer/demultiplexer 151 to the first opticalamplifier 55, whereby the signal is amplified by the first opticalamplifier 55. The amplified signal is then transmitted through the firstoptical isolator 91 to the second optical multiplexer/demultiplexer 152.Since the second optical multiplexer/demultiplexer 152 has amultiplexing function, the amplified signal is transmitted through thesecond optical transmission line 132 to the opticalmultiplexer/demultiplexer 141, whereby the signal is multiplexed withother signal to output an output signal from the output port 120. Thesecond input signal is transmitted through the second opticalmultiplexer/demultiplexer 152 to the second optical amplifier 56,whereby the signal is amplified by the second optical amplifier 56. Theamplified signal is then transmitted through the second optical isolator92 to the third optical multiplexer/demultiplexer 153. Since the thirdoptical multiplexer/demultiplexer 153 has a multiplexing function, theamplified signal is transmitted through the third optical transmissionline 133 to the optical multiplexer/demultiplexer 141, whereby thesignal is multiplexed with other signal to output an output signal fromthe output port 120. The third input signal is transmitted through thethird optical multiplexer/demultiplexer 153 to the third opticalamplifier 57, whereby the signal is amplified by the third opticalamplifier 57. The amplified signal is then transmitted through the thirdoptical isolator 93 to the fourth optical multiplexer/demultiplexer 154.Since the fourth optical multiplexer/demultiplexer 154 has amultiplexing function, the amplified signal is transmitted through thefourth optical transmission line 134 to the opticalmultiplexer/demultiplexer 141, whereby the signal is multiplexed withother signal to output an output signal from the output port 120. Thefourth input signal is transmitted through the fourth opticalmultiplexer/demultiplexer 154 to the fourth optical amplifier 58,whereby the signal is amplified by the fourth optical amplifier 58. Theamplified signal is then transmitted through the fourth optical isolator94 to the first optical multiplexer/demultiplexer 151. Since the firstoptical multiplexer/demultiplexer 151 has a multiplexing function, theamplified signal is transmitted through the first optical transmissionline 131 to the optical multiplexer/demultiplexer 141, whereby thesignal is multiplexed with other signal to output an output signal fromthe output port 120.

The use of the optical wavelength-multiplexer/demultiplexer to serve asthe same function as the optical coupler reduces the optical power lossby not less than 5 dB as compared to the 1:1 optical coupler.

Seventeenth Embodiment

A seventeenth embodiment according to the present invention will bedescribed in detail with reference to FIG. 20 which is a diagramillustrative of a novel wavelength-multiplexed optical equalizer havingfour looped optical transmission paths.

The novel wavelength-multiplexed optical equalizer is structurallydifferent from the above wavelength-multiplexed optical amplifier ofFIG. 19 in view of further provision of an optical amplifier 55 on aninput port and replacing optical amplifiers by attenuators 181, 182, 183and 184.

The novel wavelength-multiplexed optical equalizer comprises an opticalmultiplexer/demultiplexer 410 having an input port 110 and an outputport 120, and first to fourth optical transmission lines 131, 132, 133and 134 connected to the optical multiplexer/demultiplexer 410. Thefirst optical transmission line 131 is provided for transmitting asignal having a wavelength of 1530 nanometers. The second opticaltransmission line 132 is provided for transmitting a signal having awavelength of 1540 nanometers. The third optical transmission line 133is provided for transmitting a signal having a wavelength of 1550nanometers. The fourth optical transmission line is provided fortransmitting a signal having a wavelength of 1560 nanometers.

The optical input signal having four wavelength compositions of 1530nanometers, 1540 nanometers, 1550 nanometers, and 1560 nanometers istransmitted from the optical transmission line 110 to the opticalmultiplexer/demultiplexer 410, so that the optical input signal iswavelength-demultiplexed by the optical multiplexer/demultiplexer 410whereby the optical input signal is divided into a first signal having awavelength of 1530 nanometers, a second signal having a wavelength of1540 nanometers, a third signal having a wavelength of 1550 nanometers,and a fourth signal having a wavelength of 1560 nanometers. The first,second, third and fourth optical signals are inputted into the first,second, third and fourth optical transmission lines 131, 132, 133 and134 respectively.

The first optical transmission line 131 has a first opticalmultiplexer/demultiplexer 151.

The first optical multiplexer/demultiplexer 151 performs a wavelengthdemultiplexing so as to divide the first optical input signal into twodifferent wavelength optical signals having 1.55 micrometers and 1.54micrometers which are outputted from two output ports. The first opticalmultiplexer/demultiplexer 151 is used in place of the optical coupler sothat the wavelength different two optical signals has a total opticalpower which is higher than the first optical signal, thereby to solve aproblem with remarkable optical power loss caused when the signal istransmitted through a plurality of optical couplers.

The second optical transmission line 132 has a second opticalmultiplexer/demultiplexer 152. The first opticalmultiplexer/demultiplexer 151 is also connected through a seriesconnection of a first optical attenuator 181 and a first isolator 91 tothe second optical multiplexer/demultiplexer 152. One of thewavelength-demultiplexed optical signals is transmitted from the firstoptical multiplexer/demultiplexer 151 through the first opticalattenuator 181 and the first isolator 91 to the second opticalmultiplexer/demultiplexer 152.

The second optical multiplexer/demultiplexer 152 performs a wavelengthdemultiplexing so as to divide the first optical input signal into twodifferent wavelength optical signals which are outputted from two outputports. The second optical multiplexer/demultiplexer 152 is used in placeof the optical coupler so that the wavelength different two opticalsignals has a total optical power which is higher than the secondoptical signal, thereby to solve a problem with remarkable optical powerloss caused when the signal is transmitted through a plurality ofoptical couplers.

The third optical transmission line 133 has a third opticalmultiplexer/demultiplexer 153. The second opticalmultiplexer/demultiplexer 152 is also connected through a seriesconnection of a second optical attenuator 182 and a second isolator 92to the third optical multiplexer/demultiplexer 153. One of thewavelength-demultiplexed optical signals is transmitted from the secondoptical multiplexer/demultiplexer 152 through the second opticalattenuator 182 and the second isolator 92 to the third opticalmultiplexer/demultiplexer 153.

The third optical multiplexer/demultiplexer 153 performs a wavelengthdemultiplexing so as to divide the first optical input signal into twodifferent wavelength optical signals which are outputted from two outputports. The third optical multiplexer/demultiplexer 153 is used in placeof the optical coupler so that the wavelength different two opticalsignals has a total optical power which is higher than the third opticalsignal, thereby to solve a problem with remarkable optical power losscaused when the signal is transmitted through a plurality of opticalcouplers.

The fourth optical transmission line 134 has a fourth opticalmultiplexer/demultiplexer 154. The third opticalmultiplexer/demultiplexer 153 is also connected through a seriesconnection of a third optical attenuator 182 and a third isolator 93 tothe fourth optical multiplexer/demultiplexer 154. One of thewavelength-demultiplexed optical signals is transmitted from the thirdoptical multiplexer/demultiplexer 153 through the third opticalattenuator 182 and the third isolator 93 to the fourth opticalmultiplexer/demultiplexer 154.

The fourth optical multiplexer/demultiplexer 154 performs a wavelengthdemultiplexing so as to divide the first optical input signal into twodifferent wavelength optical signals which are outputted from two outputports. The fourth optical multiplexer/demultiplexer 154 is used in placeof the optical coupler so that the wavelength different two opticalsignals has a total optical power which is higher than the fourthoptical signal, thereby to solve a problem with remarkable optical powerloss caused when the signal is transmitted through a plurality ofoptical couplers.

The fourth optical multiplexer/demultiplexer 154 is also connectedthrough a series connection of a fifth optical attenuator 184 and afifth optical attenuator 94 to the first opticalmultiplexer/demultiplexer 151.

The above wavelength-multiplexed optical equalizer performs signaltransmission operation.

The signal transmission operation of the wavelength-multiplexed opticalequalizer will be described. The first input signal is transmittedthrough the first optical multiplexer/demultiplexer 151 to the firstoptical attenuator 181, whereby the signal is attenuated by the firstoptical attenuator 181. The attenuated signal is then transmittedthrough the first optical isolator 91 to the second opticalmultiplexer/demultiplexer 152. Since the second opticalmultiplexer/demultiplexer 152 has a multiplexing function, theattenuated signal is transmitted through the second optical transmissionline 132 to the optical multiplexer/demultiplexer 141, whereby thesignal is multiplexed with other signal to output an output signal fromthe output port 120. The second input signal is transmitted through thesecond optical multiplexer/demultiplexer 152 to the second opticalattenuator 182, whereby the signal is attenuated by the second opticalattenuator 182. The attenuated signal is then transmitted through thesecond optical isolator 92 to the third opticalmultiplexer/demultiplexer 153. Since the third opticalmultiplexer/demultiplexer 153 has a multiplexing function, theattenuated signal is transmitted through the third optical transmissionline 133 to the optical multiplexer/demultiplexer 141, whereby thesignal is multiplexed with other signal to output an output signal fromthe output port 120. The third input signal is transmitted through thethird optical multiplexer/demultiplexer 153 to the third opticalattenuator 183, whereby the signal is attenuated by the third opticalattenuator 183. The attenuated signal is then transmitted through thethird optical isolator 93 to the fourth opticalmultiplexer/demultiplexer 154. Since the fourth opticalmultiplexer/demultiplexer 154 has a multiplexing function, theattenuated signal is transmitted through the fourth optical transmissionline 134 to the optical multiplexer/demultiplexer 141, whereby thesignal is multiplexed with other signal to output an output signal fromthe output port 120. The fourth input signal is transmitted through thefourth optical multiplexer/demultiplexer 154 to the fourth opticalattenuator 184, whereby the signal is attenuated by the fourth opticalattenuator 184. The attenuated signal is then transmitted through thefourth optical isolator 94 to the first opticalmultiplexer/demultiplexer 151. Since the first opticalmultiplexer/demultiplexer 151 has a multiplexing function, theattenuated signal is transmitted through the first optical transmissionline 131 to the optical multiplexer/demultiplexer 141, whereby thesignal is multiplexed with other signal to output an output signal fromthe output port 120.

The use of the optical wavelength-multiplexer/demultiplexer to serve asthe same function as the optical coupler reduces the optical power lossby not less than 5 dB as compared to the 1:1 optical coupler.

Eighteenth Embodiment

An eighteenth embodiment according to the present invention will bedescribed in detail with reference to FIG. 21 which is a diagramillustrative of a novel optical gate switch utilizing optical wavelengthmultiplexer/demultiplexer and an erbium doped fiber.

An optical transmission line 121 is provided for transmitting an opticalsignal having a wavelength of 1550 nanometers. The optical transmissionline 121 has first and second optical wavelengthmultiplexer/demultiplexers 155 and 156, and an erbium doped fiber 141between the first and second optical wavelengthmultiplexer/demultiplexers 155 and 156. An excitation light source 161is provided for emitting an excitation light having a wavelength of 1480nanometers. The excitation light source 161 is connected through a firstsubordinate optical transmission line 122 to the first wavelengthmultiplexer/demultiplexer 155. A second subordinate optical transmissionline 123 extends from the second optical wavelengthmultiplexer/demultiplexer 156. The excitation light is emitted from theexcitation light source 161 and then transmitted through the firstsubordinate optical transmission line 122 to the first wavelengthmultiplexer/demultiplexer 155. The excitation light is multiplexed withthe optical signal by the first wavelength multiplexer/demultiplexer 155and further fed to the erbium doped fiber 141 to excite the erbium dopedfiber 141, whereby the optical signal transmitted on the opticaltransmission line 121 is amplified by the erbium doped fiber 141 andthen amplified signal is transmitted to the second wavelengthmultiplexer/demultiplexer 156. The excitation of the erbium doped fiber141 is caused by absorption of the 1480 nanometers wavelengthcomposition of the multiplexed signal into the erbium doped fiber 141.The 1480 nanometers wavelength composition of the multiplexed signal maypartially be unabsorbed into the erbium doped fiber 141. The multiplexedsignal is then transmitted to the second wavelengthmultiplexer/demultiplexer 156 so that the remaining 1480 nanometerswavelength composition is demultiplexed from the 1550 nanometerswavelength composition by the second wavelengthmultiplexer/demultiplexer 156, whereby the remaining 1480 nanometerswavelength composition is transmitted through the second subordinateoptical transmission line 123 whilst the 1550 nanometers wavelengthcomposition is then transmitted through the optical transmission line121.

The excitation light has a large intensity for causing an excitation ofthe erbium doped fiber 141. A majority part of the excitation light isabsorbed into the erbium doped fiber 141 for excitation of the erbiumdoped fiber 141, whilst a minority part of the excitation light is notabsorbed into the erbium doped fiber 141 and then transmitted throughthe erbium doped fiber 141. This transmitted excitation light remains tohave the large intensity. Actually, this remaining excitation light ismultiplexed with the optical signal. However, the second wavelengthmultiplexer/demultiplexer 156 is operated for demultiplexing the signalinto the optical signal having the wavelength of 1550 nanometers and theremaining excitation light having the wavelength of 1480 nanometers,whereby the second wavelength multiplexer/demultiplexer 156 sends theoptical signal having the wavelength of 1550 nanometers on the opticaltransmission line 121 and also sends the remaining excitation lighthaving the wavelength of 1480 nanometers on the second subordinateoptical transmission line 123 to avoid the transmission of the remainingexcitation light on the optical transmission line 121. The opticaltransmission line 121 may be connected to an opticalmultiplexer/demultiplexer. However, the opticalmultiplexer/demultiplexer receives no excitation light, whereby theoptical multiplexer/demultiplexer is free from any damage by theexcitation light. If the optical transmission line 121 is connected toother optical device, then the optical device receives no excitationlight, whereby the optical device is free from any damage by theexcitation light.

Nineteenth Embodiment

A nineteenth embodiment according to the present invention will bedescribed in detail with reference to FIG. 22 which is a diagramillustrative of a novel optical gate switch utilizing optical wavelengthmultiplexer/demultiplexer and an erbium doped fiber.

An optical transmission line 121 is provided for transmitting an opticalsignal having a wavelength of 1550 nanometers. The optical transmissionline 121 has first and second optical wavelengthmultiplexer/demultiplexers 157 and 158, and an erbium doped fiber 141between the first and second optical wavelengthmultiplexer/demultiplexers 157 and 158. An excitation light source 163is provided for emitting an excitation light having a wavelength of 1480nanometers. The excitation light source 163 is connected through a firstsubordinate optical transmission line 122 to the first wavelengthmultiplexer/demultiplexer 157. A second subordinate optical transmissionline 123 extends from the second optical wavelengthmultiplexer/demultiplexer 158. The second subordinate opticaltransmission line 123 has an optical reflective mirror 25 and a monitor200. The optical reflecting mirror may comprise a wavelength bandselective optical reflecting mirror which is capable of selecting awavelength band of a light to be reflected. The excitation light isemitted from the excitation light source 163 and then transmittedthrough the first subordinate optical transmission line 122 to the firstwavelength multiplexer/demultiplexer 157. The excitation light ismultiplexed with the optical signal by the first wavelengthmultiplexer/demultiplexer 157 and further fed to the erbium doped fiber141 to excite the erbium doped fiber 141, whereby the optical signaltransmitted on the optical transmission line 121 is amplified by theerbium doped fiber 141 and then amplified signal is transmitted to thesecond wavelength multiplexer/demultiplexer 158. The excitation of theerbium doped fiber 141 is caused by absorption of the 1480 nanometerswavelength composition of the multiplexed signal into the erbium dopedfiber 141. The 1480 nanometers wavelength composition of the multiplexedsignal may partially be unabsorbed into the erbium doped fiber 141. Themultiplexed signal is then transmitted to the second wavelengthmultiplexer/demultiplexer 158 so that the remaining 1480 nanometerswavelength composition is demultiplexed from the 1550 nanometerswavelength composition by the second wavelengthmultiplexer/demultiplexer 158, whereby the remaining 1480 nanometerswavelength composition is transmitted through the second subordinateoptical transmission line 123 whilst the 1550 nanometers wavelengthcomposition is then transmitted through the optical transmission line121. The remaining 1480 nanometers wavelength composition corresponds toa transmitted minority part of the excitation light having a largeintensity, for which reason the transmitted minority part of theexcitation light having a large intensity is transmitted through thesecond subordinate optical transmission line to the wavelength-bandoptical reflecting mirror 25. The transmitted minority part of theexcitation light is thus reflected by the wavelength-band opticalreflecting mirror 25 and then transmitted through the second wavelengthmultiplexer/demultiplexer 158 to the erbium doped fiber 141 againwhereby the transmitted minority part of the excitation light is furtherused to excite the erbium doped fiber 141. As a result, the efficiencyof the excitation of the erbium doped fiber 141 is high.

The excitation light has a large intensity for causing an excitation ofthe erbium doped fiber 141. A majority part of the excitation light isabsorbed into the erbium doped fiber 141 for excitation of the erbiumdoped fiber 141, whilst a minority part of the excitation light is notabsorbed into the erbium doped fiber 141 and then transmitted throughthe erbium doped fiber 141. This transmitted excitation light remains tohave the large intensity. Actually, this remaining excitation light ismultiplexed with the optical signal. However, the second wavelengthmultiplexer/demultiplexer 158 is operated for demultiplexing the signalinto the optical signal having the wavelength of 1550 nanometers and theremaining excitation light having the wavelength of 1480 nanometers,whereby the second wavelength multiplexer/demultiplexer 158 sends theoptical signal having the wavelength of 1550 nanometers on the opticaltransmission line 121 and also sends the remaining excitation lighthaving the wavelength of 1480 nanometers on the second subordinateoptical transmission line 123 to avoid the transmission of the remainingexcitation light on the optical transmission line 121. The opticaltransmission line 121 may be connected to an opticalmultiplexer/demultiplexer. However, the opticalmultiplexer/demultiplexer receives no excitation light, whereby theoptical multiplexer/demultiplexer is free from any damage by theexcitation light. If the optical transmission line 121 is connected toother optical device, then the optical device receives no excitationlight, whereby the optical device is free from any damage by theexcitation light.

Further, a slight amount of the optical signal is transmitted throughthe second wavelength multiplexer/demultiplexer 158 to the secondsubordinate optical transmission line 123. Since the wavelength-bandoptical reflecting mirror 25 sets the reflecting wavelength band at 1448nanometers for reflecting the excitation light component, then theslight amount of the optical signal having the wavelength of 1550nanometers is transmitted through the wavelength-band optical reflectingmirror 25 to the monitor 200. The monitor 200 monitors the intensity ofthe leaked optical signal for controlling optical power levels anddevice damage monitoring.

Twentieth Embodiment

A twentieth embodiment according to the present invention will bedescribed in detail with reference to FIG. 23 which is a diagramillustrative of a novel optical gate switch utilizing optical wavelengthmultiplexer/demultiplexer and an erbium doped fiber.

An optical transmission line 121 is provided for transmitting an opticalsignal having a wavelength of 1550 nanometers. The optical transmissionline 121 has first and second optical wavelengthmultiplexer/demultiplexers 157 and 158, and an erbium doped fiber 141between the first and second optical wavelengthmultiplexer/demultiplexers 157 and 158. A first excitation light source163 is provided for emitting a first excitation light having awavelength of 1480 nanometers. The first excitation light source 163 isconnected through a first subordinate optical transmission line 122 tothe first wavelength multiplexer/demultiplexer 157. A second excitationlight source 164 is provided for emitting a second excitation lighthaving a wavelength of 1480 nanometers. The second excitation lightsource 164 is connected through a second subordinate opticaltransmission line 123 to the second wavelength multiplexer/demultiplexer158. The first excitation light is emitted from the excitation lightsource 163 and then transmitted through the first subordinate opticaltransmission line 122 to the first wavelength multiplexer/demultiplexer157. The first excitation light is multiplexed with the optical signalby the first wavelength multiplexer/demultiplexer 157 and further fed tothe erbium doped fiber 141 to excite the erbium doped fiber 141, wherebythe optical signal transmitted on the optical transmission line 121 isamplified by the erbium doped fiber 141 and then amplified signal istransmitted to the second wavelength multiplexer/demultiplexer 158. Thesecond excitation light is emitted from the excitation light source 164and then transmitted through the second subordinate optical transmissionline 123 to the second wavelength multiplexer/demultiplexer 158. Thesecond excitation light is multiplexed with the optical signal by thefirst wavelength multiplexer/demultiplexer 158 and further fed to theerbium doped fiber 141 to excite the erbium doped fiber 141, whereby theoptical signal transmitted on the optical transmission line 121 isamplified by the erbium doped fiber 141 and then amplified signal istransmitted to the second wavelength multiplexer/demultiplexer 158.

The first excitation light has a large intensity for causing anexcitation of the erbium doped fiber 141. A majority part of the firstexcitation light is absorbed into the erbium doped fiber 141 forexcitation of the erbium doped fiber 141, whilst a minority part of thefirst excitation light is not absorbed into the erbium doped fiber 141and then transmitted through the erbium doped fiber 141. Thistransmitted first excitation light remains to have the large intensity.Actually, this remaining first excitation light is multiplexed with theoptical signal. However, the second wavelength multiplexer/demultiplexer158 is operated for demultiplexing the signal into the optical signalhaving the wavelength of 1550 nanometers and the remaining firstexcitation light having the wavelength of 1480 nanometers, whereby thesecond wavelength multiplexer/demultiplexer 158 sends the optical signalhaving the wavelength of 1550 nanometers on the optical transmissionline 121 and also sends the remaining first excitation light having thewavelength of 1480 nanometers on the second subordinate opticaltransmission line 123 to avoid the transmission of the remaining firstexcitation light on the optical transmission line 121. The opticaltransmission line 121 may be connected to an opticalmultiplexer/demultiplexer. However, the opticalmultiplexer/demultiplexer receives no first excitation light, wherebythe optical multiplexer/demultiplexer is free from any damage by thefirst excitation light. If the optical transmission line 121 isconnected to other optical device, then the optical device receives nofirst excitation light, whereby the optical device is free from anydamage by the first excitation light.

The second excitation light has a large intensity for causing anexcitation of the erbium doped fiber 141. A majority part of the secondexcitation light is absorbed into the erbium doped fiber 141 forexcitation of the erbium doped fiber 141, whilst a minority part of thesecond excitation light is not absorbed into the erbium doped fiber 141and then transmitted through the erbium doped fiber 141. Thistransmitted second excitation light remains to have the large intensity.Actually, this remaining second excitation light is multiplexed with theoptical signal. However, the first wavelength multiplexer/demultiplexer157 is operated for demultiplexing the signal into the optical signalhaving the wavelength of 1550 nanometers and the remaining secondexcitation light having the wavelength of 1480 nanometers, whereby thefirst wavelength multiplexer/demultiplexer 157 sends the optical signalhaving the wavelength of 1550 nanometers on the optical transmissionline 121 and also sends the remaining second excitation light having thewavelength of 1480 nanometers on the second subordinate opticaltransmission line 123 to avoid the transmission of the remaining secondexcitation light on the optical transmission line 121. The opticaltransmission line 121 may be connected to an opticalmultiplexer/demultiplexer.

However, the optical multiplexer/demultiplexer receives no secondexcitation light, whereby the optical multiplexer/demultiplexer is freefrom any damage by the second excitation light. If the opticaltransmission line 121 is connected to other optical device, then theoptical device receives no second excitation light, whereby the opticaldevice is free from any damage by the second excitation light.

The use of the first and second excitation light sources reduces therequired intensity of the individual excitation light.

Twenty First Embodiment

A twenty first embodiment according to the present invention will bedescribed in detail with reference to FIG. 24 which is a novelwavelength-multiplexed optical add-drop multiplexer including the aboveoptical multiplexer/demultiplexers utilizing the above novel opticalgate switches having the same structure as illustrated in FIG. 22,wherein band-pass filters 171 and 172 are used in place of wavelengthband selective optical reflecting mirror 25 of FIG. 22 as well asutilizing the above novel optical gate switches having the samestructure as illustrated in FIG. 21.

Twenty Second Embodiment

A twenty second embodiment according to the present invention will bedescribed in detail with reference to FIG. 25 which is a novelwavelength-multiplexed optical add-drop multiplexer which is modifiedfrom the above novel wavelength-multiplexed optical add-drop multiplexerof FIG. 15 by utilizing the above novel optical gate switches having thesame structure as illustrated in FIG. 21.

Twenty Fourth Embodiment

A twenty fourth embodiment according to the present invention will bedescribed in detail with reference to FIG. 26 which is a novelwavelength-multiplexed optical add-drop multiplexer which is modifiedfrom the above novel wavelength-multiplexed optical add-drop multiplexerof FIG. 16 by utilizing the above novel optical gate switches having thesame structure as illustrated in FIG. 22, wherein band-pass filters 171and 172 are used in place of wavelength band selective opticalreflecting mirror 25 of FIG. 22 as well as utilizing the above noveloptical gate switches having the same structure as illustrated in FIG.21.

Twenty Fourth Embodiment

A twenty fourth embodiment according to the present invention will bedescribed in detail with reference to FIG. 27 which is a novelwavelength-multiplexed optical add-drop multiplexer utilizing the abovenovel optical gate switches having the same structure as illustrated inFIG. 22, wherein band-pass filters 171 and 172 are used in place ofwavelength band selective optical reflecting mirror 25 of FIG. 22.

Twenty Fifth Embodiment

A twenty fifth embodiment according to the present invention will bedescribed in detail with reference to FIG. 28 which is a novelwavelength-multiplexed optical add-drop multiplexer which is modifiedfrom the above novel wavelength-multiplexed optical add-drop multiplexerof FIG. 15 by eliminating optical transmitters and replacing opticalreceivers by a combination of band pass filters 171, 172, 173 and 174with a monitor 200.

Twenty Sixth Embodiment

A twenty sixth embodiment according to the present invention will bedescribed in detail with reference to FIG. 29 which is a novelwavelength-multiplexed optical add-drop multiplexer which is modifiedfrom the above novel wavelength-multiplexed optical add-drop multiplexerof FIG. 19 by replacing optical amplifiers with the above novel opticalgate switches having the same structure as illustrated in FIG. 22,wherein band-pass filters 171 and 172 are used in place of wavelengthband selective optical reflecting mirror 25 of FIG. 22.

Whereas modifications of the present invention will be apparent to aperson having ordinary skill in the art, to which the inventionpertains, it is to be understood that embodiments as shown and describedby way of illustrations are by no means intended to be considered in alimiting sense. Accordingly, it is to be intended to cover by claims allmodifications which fall within the spirit and scope of the presentinvention.

1. An optical loop-structured circuit having at least a plurality of looped optical transmission lines having at least a plurality of optical transmission line junctions from which at least three optical transmission lines extend, wherein at least one of said plurality of optical transmission line junctions has an optical device configured for performing at least a wavelength demultiplexing function, which is connected to said at least three optical transmission lines, so that said optical device serves as a same role as an optical coupler so as to reduce an optical power loss as compared to a 1:1 optical coupler when an optical signal is transmitted through one said optical transmission line junction, wherein at least two of said plurality of looped optical transmission lines are connected to an optical multiplexer/demultiplexer, whilst a single looped optical transmission line is separated by said at least two of said plurality of looped optical transmission lines from said optical multiplexer/demultiplexer, so that optical signals are individually transmitted along said plurality of looped optical transmission lines, and wherein all of said plurality of optical transmission line junctions have said optical devices.
 2. The optical loop-structured circuit as claimed in claim 1, wherein, all of said plurality of optical transmission line junctions have said optical devices, and said optical device reduces the optical power loss by not less than 5 dB as compared to the 1:1 optical coupler.
 3. The optical loop-structured circuit as claimed in claim 1, wherein at least one of said plurality of looped optical transmission lines has at least a single set of an optical amplifier and an optical isolator so that said optical loop-structured circuit has a function of an optical amplifier.
 4. The optical loop-structured circuit as claimed in claim 3, wherein said at least one of said plurality of looped optical transmission lines is further connected to at least two set of an optical receiver and an optical transmitter so that said optical loop-structured circuit has a function of an optical add-drop multiplexer.
 5. The optical loop-structured circuit as claimed in claim 1, wherein at least one of said plurality of looped optical transmission lines has at least a single set of an optical attenuator and an optical isolator so that said optical loop-structured circuit has a function of an optical equalizer.
 6. The optical loop-structured circuit as claimed in claim 1, wherein each of said plurality of looped optical transmission lines has at least a single set of an optical amplifier and an optical isolator so that said optical loop-structured circuit has a function of an optical amplifier.
 7. The optical loop-structured circuit as claimed in claim 6, wherein each of said plurality of looped optical transmission lines is further connected to at least two set of an optical receiver and an optical transmitter so that said optical loop-structured circuit has a function of an optical add-drop multiplexer.
 8. The optical loop-structured circuit as claimed in claim 1, wherein each of said plurality of looped optical transmission lines has at least a single set of an optical attenuator and an optical isolator so that said optical loop-structured circuit has a function of an optical equalizer.
 9. The optical loop-structured circuit as claimed in claim 1, wherein said optical device comprises an optical multiplexer/demultiplexer.
 10. The optical loop-structured circuit as claimed in claim 1, wherein said optical device comprises an optical multiplexer.
 11. The optical loop-structured circuit as claimed in claim 1, wherein said optical device comprises an optical demultiplexer.
 12. An optical loop-structured circuit having at least a plurality of looped optical transmission lines having at least a plurality of optical transmission line junctions from which at least three optical transmission lines extend, wherein at least one of said plurality of optical transmission line junctions has an optical circulator, which is connected to said at least three optical transmission lines, so that said optical circulator serves as a same role as an optical coupler so as to reduce an optical power loss when an optical signal is transmitted through one said optical transmission line junction, wherein at least two of said plurality of looped optical transmission lines are connected to an optical multiplexer/demultiplexer, whilst a single looped optical transmission line is separated by said at least two of said plurality of looped optical transmission lines from said optical multiplexer/demultiplexer, so that optical signals are individually transmitted along said plurality of looped optical transmission lines, and wherein all of said plurality of optical transmission line junctions have said optical circulators.
 13. The optical loop-structured circuit as claimed in claim 12, wherein all of said plurality of optical transmission line junctions have said optical circulators.
 14. The optical loop-structured circuit as claimed in claim 12, wherein at least one of said plurality of looped optical transmission lines has at least a single set of an optical amplifier and an optical isolator so that said optical loop-structured circuit has a function of an optical amplifier.
 15. The optical loop-structured circuit as claimed in claim 14, wherein said at least one of said plurality of looped optical transmission lines is further connected to at least two set of an optical receiver and an optical transmitter so that said optical loop-structured circuit has a function of an optical add-drop multiplexer.
 16. The optical loop-structured circuit as claimed in claim 12, wherein at least one of said plurality of looped optical transmission lines has at least a single set of an optical attenuator and an optical isolator so that said optical loop-structured circuit has a function of an optical equalizer.
 17. The optical loop-structured circuit as claimed in claim 12, wherein each of said plurality of looped optical transmission lines has at least a single set of an optical amplifier and an optical isolator so that said optical loop-structured circuit has a function of an optical amplifier.
 18. The optical loop-structured circuit as claimed in claim 17, wherein each of said plurality of looped optical transmission lines is further connected to at least two set of an optical receiver and an optical transmitter so that said optical loop-structured circuit has a function of an optical add-drop multiplexer.
 19. The optical loop-structured circuit as claimed in claim 12, wherein each of said plurality of looped optical transmission lines has at least a single set of an optical attenuator and an optical isolator so that said optical loop-structured circuit has a function of an optical equalizer.
 20. An optical loop-structured circuit having at least a plurality of looped optical transmission lines having at least a plurality of optical transmission line junctions from which at least three optical transmission lines extend, wherein at least one of said plurality of optical transmission line junctions has an optical device configured for performing at least a wavelength demultiplexing function, which is connected to said at least three optical transmission lines, so that said optical device serves as a same role as an optical coupler so as to reduce an optical power loss as compared to a 1:1 optical coupler when an optical signal is transmitted through one said optical transmission line junction, wherein, each said optical device comprises a three port optical multiplexer/demultiplexer that demultiplexes a signal accepted at a first port connected to the optical transmission line connected into two different wavelength optical signals available respectively at a second port and a third port, the second port is connected to input to an optical isolator, the third port is connected to input to an ON-OFF amplifier, and a total power of the two different wavelength optical signals is greater than a total optical power of the signal accepted at the first port connected to the optical line connection to the master multiplexer/demultiplexer.
 21. A wavelength-multiplexed optical add-drop circuit, comprising: a master optical multiplexer/demultiplexer having a signal input port and a signal output port; and plural looped optical transmission paths connecting to the master multiplexer/demultiplexer, each optical transmission path comprising an optical line connected to the master multiplexer/demultiplexer, a three port optical device connected to the optical line, and an on/off switchable amplifier connected to an output of the three port optical device, an output of the switchable amplifier of a first one of the optical transmission paths feeding into an input of the three port optical device of a second one of the optical transmission paths, an output of the switchable amplifier of a last of the optical transmission paths feeding into an input of the three port optical device of the first one of the optical transmission paths, wherein, operation as an add-drop multiplexer is accomplished by switching the switchable amplifiers selectively ON and OFF.
 22. The circuit of claim 21, wherein, said three port optical device reduces the optical power loss by not less than 5 dB as compared to a 1:1 optical coupler.
 23. The circuit of claim 21, wherein, each three port optical device comprises an optical multiplexer/demultiplexer that demultiplexes a signal accepted at a first port connected to the optical line connected to the master multiplexer/demultiplexer into two different wavelength optical signals available respectively at a second port and a third port, the second port is connected to input to an optical isolator, the third port is connected to input to the associated on/off switchable amplifier, and a total power of the two different wavelength optical signals is greater than a total optical power of the signal accepted at the first port connected to the optical line connection to the master multiplexer/demultiplexer.
 24. The circuit of claim 23, wherein, each optical isolator is located at an output of one on/off switchable amplifier.
 25. The circuit of claim 24, wherein, each optical line of each transmission path further comprises i) a first receiver side optical coupler, an optical receiver connected to the first receiver side optical coupler, a first transmitter side optical coupler, and an optical transmitter connected to the first transmitter side optical coupler.
 26. The circuit of claim 25, wherein, an input signal is demultiplexed by master multiplexer/demultiplexer so that input signals of different wavelengths are transmitted on each transmission path and via each first receiver side optical coupler into each optical receiver, switching OFF the switchable amplifier of the first transmission path discontinues transmission through the input of the three port optical device of the second optical transmission path and a first substitute signal is transmitted from optical transmitter of the second optical transmission path so that the first substitute signal is then transmitted through the second optical transmission line to the master optical multiplexer/demultiplexer where the first substitute signal is multiplexed to output the signal output signal of the master multiplexer/demultiplexer.
 27. The circuit of claim 21, wherein, each three port optical device comprises an optical circulator that accepts at a first signal at a first port connected to the optical line connected to the master multiplexer/demultiplexer, accepts a second signal at a second port that is fed by one of the switchable amplifiers, and outputs a third signal at a third port that feeds another optical circulator.
 28. The circuit of claim 27, wherein, each optical line of each transmission path further comprises i) a first receiver side optical coupler, an optical receiver connected to the first receiver side optical coupler, a first transmitter side optical coupler, and an optical transmitter connected to the first transmitter side optical coupler, an input signal is demultiplexed by master multiplexer/demultiplexer so that input signals of different wavelengths are transmitted on each transmission path and via each first receiver side optical coupler into each optical receiver, and switching OFF the switchable amplifier of the first transmission path discontinues transmission from the third port output, of the circulator of the first optical transmission path to the second port input of the circulator of the second optical transmission path and a first substitute signal is transmitted from the optical transmitter of the second optical transmission path so that the first substitute signal is then transmitted through the second optical transmission line to the master optical multiplexer/demultiplexer where the first substitute signal is multiplexed to output the signal output signal of the master multiplexer/demultiplexer. 